xref: /petsc/src/mat/impls/shell/shell.c (revision db77db73299823266fc3e7c40818affe792d6eba)

/*
   This provides a simple shell for Fortran (and C programmers) to
  create a very simple matrix class for use with KSP without coding
  much of anything.
*/

#include <petsc/private/matimpl.h>        /*I "petscmat.h" I*/
#include <petsc/private/vecimpl.h>

struct _MatShellOps {
  /*  3 */ PetscErrorCode (*mult)(Mat,Vec,Vec);
  /*  5 */ PetscErrorCode (*multtranspose)(Mat,Vec,Vec);
  /* 17 */ PetscErrorCode (*getdiagonal)(Mat,Vec);
  /* 43 */ PetscErrorCode (*copy)(Mat,Mat,MatStructure);
  /* 60 */ PetscErrorCode (*destroy)(Mat);
};

typedef struct {
  struct _MatShellOps ops[1];

  PetscScalar vscale,vshift;
  Vec         dshift;
  Vec         left,right;
  Vec         left_work,right_work;
  Vec         left_add_work,right_add_work;
  Mat         axpy;
  PetscScalar axpy_vscale;
  PetscBool   managescalingshifts;                   /* The user will manage the scaling and shifts for the MATSHELL, not the default */
  /* support for ZeroRows/Columns operations */
  IS          zrows;
  IS          zcols;
  Vec         zvals;
  Vec         zvals_w;
  VecScatter  zvals_sct_r;
  VecScatter  zvals_sct_c;
  void        *ctx;
} Mat_Shell;


/*
     Store and scale values on zeroed rows
     xx = [x_1, 0], 0 on zeroed columns
*/
static PetscErrorCode MatShellPreZeroRight(Mat A,Vec x,Vec *xx)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  *xx = x;
  if (shell->zrows) {
    ierr = VecSet(shell->zvals_w,0.0);CHKERRQ(ierr);
    ierr = VecScatterBegin(shell->zvals_sct_c,x,shell->zvals_w,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
    ierr = VecScatterEnd(shell->zvals_sct_c,x,shell->zvals_w,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
    ierr = VecPointwiseMult(shell->zvals_w,shell->zvals_w,shell->zvals);CHKERRQ(ierr);
  }
  if (shell->zcols) {
    if (!shell->right_work) {
      ierr = MatCreateVecs(A,&shell->right_work,NULL);CHKERRQ(ierr);
    }
    ierr = VecCopy(x,shell->right_work);CHKERRQ(ierr);
    ierr = VecISSet(shell->right_work,shell->zcols,0.0);CHKERRQ(ierr);
    *xx  = shell->right_work;
  }
  PetscFunctionReturn(0);
}

/* Insert properly diagonally scaled values stored in MatShellPreZeroRight */
static PetscErrorCode MatShellPostZeroLeft(Mat A,Vec x)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (shell->zrows) {
    ierr = VecScatterBegin(shell->zvals_sct_r,shell->zvals_w,x,INSERT_VALUES,SCATTER_REVERSE);CHKERRQ(ierr);
    ierr = VecScatterEnd(shell->zvals_sct_r,shell->zvals_w,x,INSERT_VALUES,SCATTER_REVERSE);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

/*
     Store and scale values on zeroed rows
     xx = [x_1, 0], 0 on zeroed rows
*/
static PetscErrorCode MatShellPreZeroLeft(Mat A,Vec x,Vec *xx)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  *xx = NULL;
  if (!shell->zrows) {
    *xx = x;
  } else {
    if (!shell->left_work) {
      ierr = MatCreateVecs(A,NULL,&shell->left_work);CHKERRQ(ierr);
    }
    ierr = VecCopy(x,shell->left_work);CHKERRQ(ierr);
    ierr = VecSet(shell->zvals_w,0.0);CHKERRQ(ierr);
    ierr = VecScatterBegin(shell->zvals_sct_r,shell->zvals_w,shell->left_work,INSERT_VALUES,SCATTER_REVERSE);CHKERRQ(ierr);
    ierr = VecScatterEnd(shell->zvals_sct_r,shell->zvals_w,shell->left_work,INSERT_VALUES,SCATTER_REVERSE);CHKERRQ(ierr);
    ierr = VecScatterBegin(shell->zvals_sct_r,x,shell->zvals_w,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
    ierr = VecScatterEnd(shell->zvals_sct_r,x,shell->zvals_w,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
    ierr = VecPointwiseMult(shell->zvals_w,shell->zvals_w,shell->zvals);CHKERRQ(ierr);
    *xx  = shell->left_work;
  }
  PetscFunctionReturn(0);
}

/* Zero zero-columns contributions, sum contributions from properly scaled values stored in MatShellPreZeroLeft */
static PetscErrorCode MatShellPostZeroRight(Mat A,Vec x)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (shell->zcols) {
    ierr = VecISSet(x,shell->zcols,0.0);CHKERRQ(ierr);
  }
  if (shell->zrows) {
    ierr = VecScatterBegin(shell->zvals_sct_c,shell->zvals_w,x,ADD_VALUES,SCATTER_REVERSE);CHKERRQ(ierr);
    ierr = VecScatterEnd(shell->zvals_sct_c,shell->zvals_w,x,ADD_VALUES,SCATTER_REVERSE);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

/*
      xx = diag(left)*x
*/
static PetscErrorCode MatShellPreScaleLeft(Mat A,Vec x,Vec *xx)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  *xx = NULL;
  if (!shell->left) {
    *xx = x;
  } else {
    if (!shell->left_work) {ierr = VecDuplicate(shell->left,&shell->left_work);CHKERRQ(ierr);}
    ierr = VecPointwiseMult(shell->left_work,x,shell->left);CHKERRQ(ierr);
    *xx  = shell->left_work;
  }
  PetscFunctionReturn(0);
}

/*
     xx = diag(right)*x
*/
static PetscErrorCode MatShellPreScaleRight(Mat A,Vec x,Vec *xx)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  *xx = NULL;
  if (!shell->right) {
    *xx = x;
  } else {
    if (!shell->right_work) {ierr = VecDuplicate(shell->right,&shell->right_work);CHKERRQ(ierr);}
    ierr = VecPointwiseMult(shell->right_work,x,shell->right);CHKERRQ(ierr);
    *xx  = shell->right_work;
  }
  PetscFunctionReturn(0);
}

/*
    x = diag(left)*x
*/
static PetscErrorCode MatShellPostScaleLeft(Mat A,Vec x)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (shell->left) {ierr = VecPointwiseMult(x,x,shell->left);CHKERRQ(ierr);}
  PetscFunctionReturn(0);
}

/*
    x = diag(right)*x
*/
static PetscErrorCode MatShellPostScaleRight(Mat A,Vec x)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (shell->right) {ierr = VecPointwiseMult(x,x,shell->right);CHKERRQ(ierr);}
  PetscFunctionReturn(0);
}

/*
         Y = vscale*Y + diag(dshift)*X + vshift*X

         On input Y already contains A*x
*/
static PetscErrorCode MatShellShiftAndScale(Mat A,Vec X,Vec Y)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (shell->dshift) {          /* get arrays because there is no VecPointwiseMultAdd() */
    PetscInt          i,m;
    const PetscScalar *x,*d;
    PetscScalar       *y;
    ierr = VecGetLocalSize(X,&m);CHKERRQ(ierr);
    ierr = VecGetArrayRead(shell->dshift,&d);CHKERRQ(ierr);
    ierr = VecGetArrayRead(X,&x);CHKERRQ(ierr);
    ierr = VecGetArray(Y,&y);CHKERRQ(ierr);
    for (i=0; i<m; i++) y[i] = shell->vscale*y[i] + d[i]*x[i];
    ierr = VecRestoreArrayRead(shell->dshift,&d);CHKERRQ(ierr);
    ierr = VecRestoreArrayRead(X,&x);CHKERRQ(ierr);
    ierr = VecRestoreArray(Y,&y);CHKERRQ(ierr);
  } else {
    ierr = VecScale(Y,shell->vscale);CHKERRQ(ierr);
  }
  if (shell->vshift != 0.0) {ierr = VecAXPY(Y,shell->vshift,X);CHKERRQ(ierr);} /* if test is for non-square matrices */
  PetscFunctionReturn(0);
}

PetscErrorCode  MatShellGetContext_Shell(Mat mat,void *ctx)
{
  PetscFunctionBegin;
  *(void**)ctx = ((Mat_Shell*)(mat->data))->ctx;
  PetscFunctionReturn(0);
}

/*@
    MatShellGetContext - Returns the user-provided context associated with a shell matrix.

    Not Collective

    Input Parameter:
.   mat - the matrix, should have been created with MatCreateShell()

    Output Parameter:
.   ctx - the user provided context

    Level: advanced

   Fortran Notes:
    To use this from Fortran you must write a Fortran interface definition for this
    function that tells Fortran the Fortran derived data type that you are passing in as the ctx argument.

.seealso: MatCreateShell(), MatShellSetOperation(), MatShellSetContext()
@*/
PetscErrorCode  MatShellGetContext(Mat mat,void *ctx)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(mat,MAT_CLASSID,1);
  PetscValidPointer(ctx,2);
  ierr = PetscUseMethod(mat,"MatShellGetContext_C",(Mat,void*),(mat,ctx));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

static PetscErrorCode MatZeroRowsColumns_Local_Shell(Mat mat,PetscInt nr,PetscInt rows[],PetscInt nc,PetscInt cols[],PetscScalar diag,PetscBool rc)
{
  PetscErrorCode ierr;
  Mat_Shell      *shell = (Mat_Shell*)mat->data;
  Vec            x = NULL,b = NULL;
  IS             is1, is2;
  const PetscInt *ridxs;
  PetscInt       *idxs,*gidxs;
  PetscInt       cum,rst,cst,i;

  PetscFunctionBegin;
  if (shell->axpy) SETERRQ(PetscObjectComm((PetscObject)mat),PETSC_ERR_SUP,"Cannot diagonal scale MATSHELL after MatAXPY operation");
  if (!shell->zvals) {
    ierr = MatCreateVecs(mat,NULL,&shell->zvals);CHKERRQ(ierr);
  }
  if (!shell->zvals_w) {
    ierr = VecDuplicate(shell->zvals,&shell->zvals_w);CHKERRQ(ierr);
  }
  ierr = MatGetOwnershipRange(mat,&rst,NULL);CHKERRQ(ierr);
  ierr = MatGetOwnershipRangeColumn(mat,&cst,NULL);CHKERRQ(ierr);

  /* Expand/create index set of zeroed rows */
  ierr = PetscMalloc1(nr,&idxs);CHKERRQ(ierr);
  for (i = 0; i < nr; i++) idxs[i] = rows[i] + rst;
  ierr = ISCreateGeneral(PETSC_COMM_SELF,nr,idxs,PETSC_OWN_POINTER,&is1);CHKERRQ(ierr);
  ierr = ISSort(is1);CHKERRQ(ierr);
  ierr = VecISSet(shell->zvals,is1,diag);CHKERRQ(ierr);
  if (shell->zrows) {
    ierr = ISSum(shell->zrows,is1,&is2);CHKERRQ(ierr);
    ierr = ISDestroy(&shell->zrows);CHKERRQ(ierr);
    ierr = ISDestroy(&is1);CHKERRQ(ierr);
    shell->zrows = is2;
  } else shell->zrows = is1;

  /* Create scatters for diagonal values communications */
  ierr = VecScatterDestroy(&shell->zvals_sct_c);CHKERRQ(ierr);
  ierr = VecScatterDestroy(&shell->zvals_sct_r);CHKERRQ(ierr);

  /* row scatter: from/to left vector */
  ierr = MatCreateVecs(mat,&x,&b);CHKERRQ(ierr);
  ierr = VecScatterCreate(b,shell->zrows,shell->zvals_w,shell->zrows,&shell->zvals_sct_r);CHKERRQ(ierr);

  /* col scatter: from right vector to left vector */
  ierr = ISGetIndices(shell->zrows,&ridxs);CHKERRQ(ierr);
  ierr = ISGetLocalSize(shell->zrows,&nr);CHKERRQ(ierr);
  ierr = PetscMalloc1(nr,&gidxs);CHKERRQ(ierr);
  for (i = 0, cum  = 0; i < nr; i++) {
    if (ridxs[i] >= mat->cmap->N) continue;
    gidxs[cum] = ridxs[i];
    cum++;
  }
  ierr = ISCreateGeneral(PetscObjectComm((PetscObject)mat),cum,gidxs,PETSC_OWN_POINTER,&is1);CHKERRQ(ierr);
  ierr = VecScatterCreate(x,is1,shell->zvals_w,is1,&shell->zvals_sct_c);CHKERRQ(ierr);
  ierr = ISDestroy(&is1);CHKERRQ(ierr);
  ierr = VecDestroy(&x);CHKERRQ(ierr);
  ierr = VecDestroy(&b);CHKERRQ(ierr);

  /* Expand/create index set of zeroed columns */
  if (rc) {
    ierr = PetscMalloc1(nc,&idxs);CHKERRQ(ierr);
    for (i = 0; i < nc; i++) idxs[i] = cols[i] + cst;
    ierr = ISCreateGeneral(PETSC_COMM_SELF,nc,idxs,PETSC_OWN_POINTER,&is1);CHKERRQ(ierr);
    ierr = ISSort(is1);CHKERRQ(ierr);
    if (shell->zcols) {
      ierr = ISSum(shell->zcols,is1,&is2);CHKERRQ(ierr);
      ierr = ISDestroy(&shell->zcols);CHKERRQ(ierr);
      ierr = ISDestroy(&is1);CHKERRQ(ierr);
      shell->zcols = is2;
    } else shell->zcols = is1;
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode MatZeroRows_Shell(Mat mat,PetscInt n,const PetscInt rows[],PetscScalar diag,Vec x,Vec b)
{
  PetscInt       nr, *lrows;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (x && b) {
    Vec          xt;
    PetscScalar *vals;
    PetscInt    *gcols,i,st,nl,nc;

    ierr = PetscMalloc1(n,&gcols);CHKERRQ(ierr);
    for (i = 0, nc = 0; i < n; i++) if (rows[i] < mat->cmap->N) gcols[nc++] = rows[i];

    ierr = MatCreateVecs(mat,&xt,NULL);CHKERRQ(ierr);
    ierr = VecCopy(x,xt);CHKERRQ(ierr);
    ierr = PetscCalloc1(nc,&vals);CHKERRQ(ierr);
    ierr = VecSetValues(xt,nc,gcols,vals,INSERT_VALUES);CHKERRQ(ierr); /* xt = [x1, 0] */
    ierr = PetscFree(vals);CHKERRQ(ierr);
    ierr = VecAssemblyBegin(xt);CHKERRQ(ierr);
    ierr = VecAssemblyEnd(xt);CHKERRQ(ierr);
    ierr = VecAYPX(xt,-1.0,x);CHKERRQ(ierr);                           /* xt = [0, x2] */

    ierr = VecGetOwnershipRange(xt,&st,NULL);CHKERRQ(ierr);
    ierr = VecGetLocalSize(xt,&nl);CHKERRQ(ierr);
    ierr = VecGetArray(xt,&vals);CHKERRQ(ierr);
    for (i = 0; i < nl; i++) {
      PetscInt g = i + st;
      if (g > mat->rmap->N) continue;
      if (PetscAbsScalar(vals[i]) == 0.0) continue;
      ierr = VecSetValue(b,g,diag*vals[i],INSERT_VALUES);CHKERRQ(ierr);
    }
    ierr = VecRestoreArray(xt,&vals);CHKERRQ(ierr);
    ierr = VecAssemblyBegin(b);CHKERRQ(ierr);
    ierr = VecAssemblyEnd(b);CHKERRQ(ierr);                            /* b  = [b1, x2 * diag] */
    ierr = VecDestroy(&xt);CHKERRQ(ierr);
    ierr = PetscFree(gcols);CHKERRQ(ierr);
  }
  ierr = PetscLayoutMapLocal(mat->rmap,n,rows,&nr,&lrows,NULL);CHKERRQ(ierr);
  ierr = MatZeroRowsColumns_Local_Shell(mat,nr,lrows,0,NULL,diag,PETSC_FALSE);CHKERRQ(ierr);
  ierr = PetscFree(lrows);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

static PetscErrorCode MatZeroRowsColumns_Shell(Mat mat,PetscInt n,const PetscInt rowscols[],PetscScalar diag,Vec x,Vec b)
{
  PetscInt       *lrows, *lcols;
  PetscInt       nr, nc;
  PetscBool      congruent;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (x && b) {
    Vec          xt, bt;
    PetscScalar *vals;
    PetscInt    *grows,*gcols,i,st,nl;

    ierr = PetscMalloc2(n,&grows,n,&gcols);CHKERRQ(ierr);
    for (i = 0, nr = 0; i < n; i++) if (rowscols[i] < mat->rmap->N) grows[nr++] = rowscols[i];
    for (i = 0, nc = 0; i < n; i++) if (rowscols[i] < mat->cmap->N) gcols[nc++] = rowscols[i];
    ierr = PetscCalloc1(n,&vals);CHKERRQ(ierr);

    ierr = MatCreateVecs(mat,&xt,&bt);CHKERRQ(ierr);
    ierr = VecCopy(x,xt);CHKERRQ(ierr);
    ierr = VecSetValues(xt,nc,gcols,vals,INSERT_VALUES);CHKERRQ(ierr); /* xt = [x1, 0] */
    ierr = VecAssemblyBegin(xt);CHKERRQ(ierr);
    ierr = VecAssemblyEnd(xt);CHKERRQ(ierr);
    ierr = VecAXPY(xt,-1.0,x);CHKERRQ(ierr);                           /* xt = [0, -x2] */
    ierr = MatMult(mat,xt,bt);CHKERRQ(ierr);                           /* bt = [-A12*x2,-A22*x2] */
    ierr = VecSetValues(bt,nr,grows,vals,INSERT_VALUES);CHKERRQ(ierr); /* bt = [-A12*x2,0] */
    ierr = VecAssemblyBegin(bt);CHKERRQ(ierr);
    ierr = VecAssemblyEnd(bt);CHKERRQ(ierr);
    ierr = VecAXPY(b,1.0,bt);CHKERRQ(ierr);                            /* b  = [b1 - A12*x2, b2] */
    ierr = VecSetValues(bt,nr,grows,vals,INSERT_VALUES);CHKERRQ(ierr); /* b  = [b1 - A12*x2, 0] */
    ierr = VecAssemblyBegin(bt);CHKERRQ(ierr);
    ierr = VecAssemblyEnd(bt);CHKERRQ(ierr);
    ierr = PetscFree(vals);CHKERRQ(ierr);

    ierr = VecGetOwnershipRange(xt,&st,NULL);CHKERRQ(ierr);
    ierr = VecGetLocalSize(xt,&nl);CHKERRQ(ierr);
    ierr = VecGetArray(xt,&vals);CHKERRQ(ierr);
    for (i = 0; i < nl; i++) {
      PetscInt g = i + st;
      if (g > mat->rmap->N) continue;
      if (PetscAbsScalar(vals[i]) == 0.0) continue;
      ierr = VecSetValue(b,g,-diag*vals[i],INSERT_VALUES);CHKERRQ(ierr);
    }
    ierr = VecRestoreArray(xt,&vals);CHKERRQ(ierr);
    ierr = VecAssemblyBegin(b);CHKERRQ(ierr);
    ierr = VecAssemblyEnd(b);CHKERRQ(ierr);                            /* b  = [b1 - A12*x2, x2 * diag] */
    ierr = VecDestroy(&xt);CHKERRQ(ierr);
    ierr = VecDestroy(&bt);CHKERRQ(ierr);
    ierr = PetscFree2(grows,gcols);CHKERRQ(ierr);
  }
  ierr = PetscLayoutMapLocal(mat->rmap,n,rowscols,&nr,&lrows,NULL);CHKERRQ(ierr);
  ierr = MatHasCongruentLayouts(mat,&congruent);CHKERRQ(ierr);
  if (congruent) {
    nc    = nr;
    lcols = lrows;
  } else { /* MatZeroRowsColumns implicitly assumes the rowscols indices are for a square matrix, here we handle a more general case */
    PetscInt i,nt,*t;

    ierr = PetscMalloc1(n,&t);CHKERRQ(ierr);
    for (i = 0, nt = 0; i < n; i++) if (rowscols[i] < mat->cmap->N) t[nt++] = rowscols[i];
    ierr = PetscLayoutMapLocal(mat->cmap,nt,t,&nc,&lcols,NULL);CHKERRQ(ierr);
    ierr = PetscFree(t);CHKERRQ(ierr);
  }
  ierr = MatZeroRowsColumns_Local_Shell(mat,nr,lrows,nc,lcols,diag,PETSC_TRUE);CHKERRQ(ierr);
  if (!congruent) {
    ierr = PetscFree(lcols);CHKERRQ(ierr);
  }
  ierr = PetscFree(lrows);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

PetscErrorCode MatDestroy_Shell(Mat mat)
{
  PetscErrorCode ierr;
  Mat_Shell      *shell = (Mat_Shell*)mat->data;

  PetscFunctionBegin;
  if (shell->ops->destroy) {
    ierr = (*shell->ops->destroy)(mat);CHKERRQ(ierr);
  }
  ierr = VecDestroy(&shell->left);CHKERRQ(ierr);
  ierr = VecDestroy(&shell->right);CHKERRQ(ierr);
  ierr = VecDestroy(&shell->dshift);CHKERRQ(ierr);
  ierr = VecDestroy(&shell->left_work);CHKERRQ(ierr);
  ierr = VecDestroy(&shell->right_work);CHKERRQ(ierr);
  ierr = VecDestroy(&shell->left_add_work);CHKERRQ(ierr);
  ierr = VecDestroy(&shell->right_add_work);CHKERRQ(ierr);
  ierr = MatDestroy(&shell->axpy);CHKERRQ(ierr);
  ierr = VecDestroy(&shell->zvals_w);CHKERRQ(ierr);
  ierr = VecDestroy(&shell->zvals);CHKERRQ(ierr);
  ierr = VecScatterDestroy(&shell->zvals_sct_c);CHKERRQ(ierr);
  ierr = VecScatterDestroy(&shell->zvals_sct_r);CHKERRQ(ierr);
  ierr = ISDestroy(&shell->zrows);CHKERRQ(ierr);
  ierr = ISDestroy(&shell->zcols);CHKERRQ(ierr);
  ierr = PetscFree(mat->data);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)mat,"MatShellGetContext_C",NULL);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)mat,"MatShellSetContext_C",NULL);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)mat,"MatShellSetVecType_C",NULL);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)mat,"MatShellSetManageScalingShifts_C",NULL);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)mat,"MatShellSetOperation_C",NULL);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)mat,"MatShellGetOperation_C",NULL);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

PetscErrorCode MatCopy_Shell(Mat A,Mat B,MatStructure str)
{
  Mat_Shell       *shellA = (Mat_Shell*)A->data,*shellB = (Mat_Shell*)B->data;
  PetscErrorCode  ierr;
  PetscBool       matflg;

  PetscFunctionBegin;
  ierr = PetscObjectTypeCompare((PetscObject)B,MATSHELL,&matflg);CHKERRQ(ierr);
  if (!matflg) SETERRQ(PetscObjectComm((PetscObject)A),PETSC_ERR_ARG_NOTSAMETYPE,"Output matrix must be a MATSHELL");

  ierr = PetscMemcpy(B->ops,A->ops,sizeof(struct _MatOps));CHKERRQ(ierr);
  ierr = PetscMemcpy(shellB->ops,shellA->ops,sizeof(struct _MatShellOps));CHKERRQ(ierr);

  if (shellA->ops->copy) {
    ierr = (*shellA->ops->copy)(A,B,str);CHKERRQ(ierr);
  }
  shellB->vscale = shellA->vscale;
  shellB->vshift = shellA->vshift;
  if (shellA->dshift) {
    if (!shellB->dshift) {
      ierr = VecDuplicate(shellA->dshift,&shellB->dshift);CHKERRQ(ierr);
    }
    ierr = VecCopy(shellA->dshift,shellB->dshift);CHKERRQ(ierr);
  } else {
    ierr = VecDestroy(&shellB->dshift);CHKERRQ(ierr);
  }
  if (shellA->left) {
    if (!shellB->left) {
      ierr = VecDuplicate(shellA->left,&shellB->left);CHKERRQ(ierr);
    }
    ierr = VecCopy(shellA->left,shellB->left);CHKERRQ(ierr);
  } else {
    ierr = VecDestroy(&shellB->left);CHKERRQ(ierr);
  }
  if (shellA->right) {
    if (!shellB->right) {
      ierr = VecDuplicate(shellA->right,&shellB->right);CHKERRQ(ierr);
    }
    ierr = VecCopy(shellA->right,shellB->right);CHKERRQ(ierr);
  } else {
    ierr = VecDestroy(&shellB->right);CHKERRQ(ierr);
  }
  ierr = MatDestroy(&shellB->axpy);CHKERRQ(ierr);
  if (shellA->axpy) {
    ierr                = PetscObjectReference((PetscObject)shellA->axpy);CHKERRQ(ierr);
    shellB->axpy        = shellA->axpy;
    shellB->axpy_vscale = shellA->axpy_vscale;
  }
  if (shellA->zrows) {
    ierr = ISDuplicate(shellA->zrows,&shellB->zrows);CHKERRQ(ierr);
    if (shellA->zcols) {
      ierr = ISDuplicate(shellA->zcols,&shellB->zcols);CHKERRQ(ierr);
    }
    ierr = VecDuplicate(shellA->zvals,&shellB->zvals);CHKERRQ(ierr);
    ierr = VecCopy(shellA->zvals,shellB->zvals);CHKERRQ(ierr);
    ierr = VecDuplicate(shellA->zvals_w,&shellB->zvals_w);CHKERRQ(ierr);
    ierr = PetscObjectReference((PetscObject)shellA->zvals_sct_r);CHKERRQ(ierr);
    ierr = PetscObjectReference((PetscObject)shellA->zvals_sct_c);CHKERRQ(ierr);
    shellB->zvals_sct_r = shellA->zvals_sct_r;
    shellB->zvals_sct_c = shellA->zvals_sct_c;
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatDuplicate_Shell(Mat mat,MatDuplicateOption op,Mat *M)
{
  PetscErrorCode ierr;
  void           *ctx;

  PetscFunctionBegin;
  ierr = MatShellGetContext(mat,&ctx);CHKERRQ(ierr);
  ierr = MatCreateShell(PetscObjectComm((PetscObject)mat),mat->rmap->n,mat->cmap->n,mat->rmap->N,mat->cmap->N,ctx,M);CHKERRQ(ierr);
  if (op != MAT_DO_NOT_COPY_VALUES) {
    ierr = MatCopy(mat,*M,SAME_NONZERO_PATTERN);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatMult_Shell(Mat A,Vec x,Vec y)
{
  Mat_Shell        *shell = (Mat_Shell*)A->data;
  PetscErrorCode   ierr;
  Vec              xx;
  PetscObjectState instate,outstate;

  PetscFunctionBegin;
  if (!shell->ops->mult) SETERRQ(PetscObjectComm((PetscObject)A),PETSC_ERR_ARG_WRONGSTATE,"Have not provided a MatMult() for this MATSHELL");
  ierr = MatShellPreZeroRight(A,x,&xx);CHKERRQ(ierr);
  ierr = MatShellPreScaleRight(A,xx,&xx);CHKERRQ(ierr);
  ierr = PetscObjectStateGet((PetscObject)y, &instate);CHKERRQ(ierr);
  ierr = (*shell->ops->mult)(A,xx,y);CHKERRQ(ierr);
  ierr = PetscObjectStateGet((PetscObject)y, &outstate);CHKERRQ(ierr);
  if (instate == outstate) {
    /* increase the state of the output vector since the user did not update its state themself as should have been done */
    ierr = PetscObjectStateIncrease((PetscObject)y);CHKERRQ(ierr);
  }
  ierr = MatShellShiftAndScale(A,xx,y);CHKERRQ(ierr);
  ierr = MatShellPostScaleLeft(A,y);CHKERRQ(ierr);
  ierr = MatShellPostZeroLeft(A,y);CHKERRQ(ierr);

  if (shell->axpy) {
    if (!shell->left_work) {ierr = MatCreateVecs(A,&shell->left_work,NULL);CHKERRQ(ierr);}
    ierr = MatMult(shell->axpy,x,shell->left_work);CHKERRQ(ierr);
    ierr = VecAXPY(y,shell->axpy_vscale,shell->left_work);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatMultAdd_Shell(Mat A,Vec x,Vec y,Vec z)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (y == z) {
    if (!shell->right_add_work) {ierr = VecDuplicate(z,&shell->right_add_work);CHKERRQ(ierr);}
    ierr = MatMult(A,x,shell->right_add_work);CHKERRQ(ierr);
    ierr = VecAXPY(z,1.0,shell->right_add_work);CHKERRQ(ierr);
  } else {
    ierr = MatMult(A,x,z);CHKERRQ(ierr);
    ierr = VecAXPY(z,1.0,y);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatMultTranspose_Shell(Mat A,Vec x,Vec y)
{
  Mat_Shell        *shell = (Mat_Shell*)A->data;
  PetscErrorCode   ierr;
  Vec              xx;
  PetscObjectState instate,outstate;

  PetscFunctionBegin;
  ierr = MatShellPreZeroLeft(A,x,&xx);CHKERRQ(ierr);
  ierr = MatShellPreScaleLeft(A,xx,&xx);CHKERRQ(ierr);
  ierr = PetscObjectStateGet((PetscObject)y, &instate);CHKERRQ(ierr);
  if (!shell->ops->multtranspose) SETERRQ(PetscObjectComm((PetscObject)A),PETSC_ERR_ARG_WRONGSTATE,"Have not provided a MatMultTranspose() for this MATSHELL");
  ierr = (*shell->ops->multtranspose)(A,xx,y);CHKERRQ(ierr);
  ierr = PetscObjectStateGet((PetscObject)y, &outstate);CHKERRQ(ierr);
  if (instate == outstate) {
    /* increase the state of the output vector since the user did not update its state themself as should have been done */
    ierr = PetscObjectStateIncrease((PetscObject)y);CHKERRQ(ierr);
  }
  ierr = MatShellShiftAndScale(A,xx,y);CHKERRQ(ierr);
  ierr = MatShellPostScaleRight(A,y);CHKERRQ(ierr);
  ierr = MatShellPostZeroRight(A,y);CHKERRQ(ierr);

  if (shell->axpy) {
    if (!shell->right_work) {ierr = MatCreateVecs(A,NULL,&shell->right_work);CHKERRQ(ierr);}
    ierr = MatMultTranspose(shell->axpy,x,shell->right_work);CHKERRQ(ierr);
    ierr = VecAXPY(y,shell->axpy_vscale,shell->right_work);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatMultTransposeAdd_Shell(Mat A,Vec x,Vec y,Vec z)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (y == z) {
    if (!shell->left_add_work) {ierr = VecDuplicate(z,&shell->left_add_work);CHKERRQ(ierr);}
    ierr = MatMultTranspose(A,x,shell->left_add_work);CHKERRQ(ierr);
    ierr = VecAXPY(z,1.0,shell->left_add_work);CHKERRQ(ierr);
  } else {
    ierr = MatMultTranspose(A,x,z);CHKERRQ(ierr);
    ierr = VecAXPY(z,1.0,y);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

/*
          diag(left)(vscale*A + diag(dshift) + vshift I)diag(right)
*/
PetscErrorCode MatGetDiagonal_Shell(Mat A,Vec v)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (shell->ops->getdiagonal) {
    ierr = (*shell->ops->getdiagonal)(A,v);CHKERRQ(ierr);
  } else SETERRQ(PetscObjectComm((PetscObject)A),PETSC_ERR_ARG_WRONGSTATE,"Must provide shell matrix with routine to return diagonal using\nMatShellSetOperation(S,MATOP_GET_DIAGONAL,...)");
  ierr = VecScale(v,shell->vscale);CHKERRQ(ierr);
  if (shell->dshift) {
    ierr = VecAXPY(v,1.0,shell->dshift);CHKERRQ(ierr);
  }
  ierr = VecShift(v,shell->vshift);CHKERRQ(ierr);
  if (shell->left)  {ierr = VecPointwiseMult(v,v,shell->left);CHKERRQ(ierr);}
  if (shell->right) {ierr = VecPointwiseMult(v,v,shell->right);CHKERRQ(ierr);}
  if (shell->zrows) {
    ierr = VecScatterBegin(shell->zvals_sct_r,shell->zvals,v,INSERT_VALUES,SCATTER_REVERSE);CHKERRQ(ierr);
    ierr = VecScatterEnd(shell->zvals_sct_r,shell->zvals,v,INSERT_VALUES,SCATTER_REVERSE);CHKERRQ(ierr);
  }
  if (shell->axpy) {
    if (!shell->left_work) {ierr = VecDuplicate(v,&shell->left_work);CHKERRQ(ierr);}
    ierr = MatGetDiagonal(shell->axpy,shell->left_work);CHKERRQ(ierr);
    ierr = VecAXPY(v,shell->axpy_vscale,shell->left_work);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatShift_Shell(Mat Y,PetscScalar a)
{
  Mat_Shell      *shell = (Mat_Shell*)Y->data;
  PetscErrorCode ierr;
  PetscBool      flg;

  PetscFunctionBegin;
  ierr = MatHasCongruentLayouts(Y,&flg);CHKERRQ(ierr);
  if (!flg) SETERRQ(PetscObjectComm((PetscObject)Y),PETSC_ERR_SUP,"Cannot shift shell matrix if it is not congruent");
  if (shell->left || shell->right) {
    if (!shell->dshift) {
      ierr = VecDuplicate(shell->left ? shell->left : shell->right, &shell->dshift);CHKERRQ(ierr);
      ierr = VecSet(shell->dshift,a);CHKERRQ(ierr);
    } else {
      if (shell->left)  {ierr = VecPointwiseMult(shell->dshift,shell->dshift,shell->left);CHKERRQ(ierr);}
      if (shell->right) {ierr = VecPointwiseMult(shell->dshift,shell->dshift,shell->right);CHKERRQ(ierr);}
      ierr = VecShift(shell->dshift,a);CHKERRQ(ierr);
    }
    if (shell->left)  {ierr = VecPointwiseDivide(shell->dshift,shell->dshift,shell->left);CHKERRQ(ierr);}
    if (shell->right) {ierr = VecPointwiseDivide(shell->dshift,shell->dshift,shell->right);CHKERRQ(ierr);}
  } else shell->vshift += a;
  if (shell->zrows) {
    ierr = VecShift(shell->zvals,a);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatDiagonalSet_Shell_Private(Mat A,Vec D,PetscScalar s)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (!shell->dshift) {ierr = VecDuplicate(D,&shell->dshift);CHKERRQ(ierr);}
  if (shell->left || shell->right) {
    if (!shell->right_work) {ierr = VecDuplicate(shell->left ? shell->left : shell->right, &shell->right_work);CHKERRQ(ierr);}
    if (shell->left && shell->right)  {
      ierr = VecPointwiseDivide(shell->right_work,D,shell->left);CHKERRQ(ierr);
      ierr = VecPointwiseDivide(shell->right_work,shell->right_work,shell->right);CHKERRQ(ierr);
    } else if (shell->left) {
      ierr = VecPointwiseDivide(shell->right_work,D,shell->left);CHKERRQ(ierr);
    } else {
      ierr = VecPointwiseDivide(shell->right_work,D,shell->right);CHKERRQ(ierr);
    }
    ierr = VecAXPY(shell->dshift,s,shell->right_work);CHKERRQ(ierr);
  } else {
    ierr = VecAXPY(shell->dshift,s,D);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatDiagonalSet_Shell(Mat A,Vec D,InsertMode ins)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;
  Vec            d;
  PetscErrorCode ierr;
  PetscBool      flg;

  PetscFunctionBegin;
  ierr = MatHasCongruentLayouts(A,&flg);CHKERRQ(ierr);
  if (!flg) SETERRQ(PetscObjectComm((PetscObject)A),PETSC_ERR_SUP,"Cannot diagonal set or shift shell matrix if it is not congruent");
  if (ins == INSERT_VALUES) {
    if (!A->ops->getdiagonal) SETERRQ(PetscObjectComm((PetscObject)A),PETSC_ERR_SUP,"Operation MATOP_GETDIAGONAL must be set first");
    ierr = VecDuplicate(D,&d);CHKERRQ(ierr);
    ierr = MatGetDiagonal(A,d);CHKERRQ(ierr);
    ierr = MatDiagonalSet_Shell_Private(A,d,-1.);CHKERRQ(ierr);
    ierr = MatDiagonalSet_Shell_Private(A,D,1.);CHKERRQ(ierr);
    ierr = VecDestroy(&d);CHKERRQ(ierr);
    if (shell->zrows) {
      ierr = VecCopy(D,shell->zvals);CHKERRQ(ierr);
    }
  } else {
    ierr = MatDiagonalSet_Shell_Private(A,D,1.);CHKERRQ(ierr);
    if (shell->zrows) {
      ierr = VecAXPY(shell->zvals,1.0,D);CHKERRQ(ierr);
    }
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatScale_Shell(Mat Y,PetscScalar a)
{
  Mat_Shell      *shell = (Mat_Shell*)Y->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  shell->vscale *= a;
  shell->vshift *= a;
  if (shell->dshift) {
    ierr = VecScale(shell->dshift,a);CHKERRQ(ierr);
  }
  shell->axpy_vscale *= a;
  if (shell->zrows) {
    ierr = VecScale(shell->zvals,a);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode MatDiagonalScale_Shell(Mat Y,Vec left,Vec right)
{
  Mat_Shell      *shell = (Mat_Shell*)Y->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (shell->axpy) SETERRQ(PetscObjectComm((PetscObject)Y),PETSC_ERR_SUP,"Cannot diagonal scale MATSHELL after MatAXPY operation");
  if (left) {
    if (!shell->left) {
      ierr = VecDuplicate(left,&shell->left);CHKERRQ(ierr);
      ierr = VecCopy(left,shell->left);CHKERRQ(ierr);
    } else {
      ierr = VecPointwiseMult(shell->left,shell->left,left);CHKERRQ(ierr);
    }
    if (shell->zrows) {
      ierr = VecPointwiseMult(shell->zvals,shell->zvals,left);CHKERRQ(ierr);
    }
  }
  if (right) {
    if (!shell->right) {
      ierr = VecDuplicate(right,&shell->right);CHKERRQ(ierr);
      ierr = VecCopy(right,shell->right);CHKERRQ(ierr);
    } else {
      ierr = VecPointwiseMult(shell->right,shell->right,right);CHKERRQ(ierr);
    }
    if (shell->zrows) {
      if (!shell->left_work) {
        ierr = MatCreateVecs(Y,NULL,&shell->left_work);CHKERRQ(ierr);
      }
      ierr = VecSet(shell->zvals_w,1.0);CHKERRQ(ierr);
      ierr = VecScatterBegin(shell->zvals_sct_c,right,shell->zvals_w,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
      ierr = VecScatterEnd(shell->zvals_sct_c,right,shell->zvals_w,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
      ierr = VecPointwiseMult(shell->zvals,shell->zvals,shell->zvals_w);CHKERRQ(ierr);
    }
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatAssemblyEnd_Shell(Mat Y,MatAssemblyType t)
{
  Mat_Shell      *shell = (Mat_Shell*)Y->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (t == MAT_FINAL_ASSEMBLY) {
    shell->vshift = 0.0;
    shell->vscale = 1.0;
    ierr = VecDestroy(&shell->dshift);CHKERRQ(ierr);
    ierr = VecDestroy(&shell->left);CHKERRQ(ierr);
    ierr = VecDestroy(&shell->right);CHKERRQ(ierr);
    ierr = MatDestroy(&shell->axpy);CHKERRQ(ierr);
    ierr = VecScatterDestroy(&shell->zvals_sct_c);CHKERRQ(ierr);
    ierr = VecScatterDestroy(&shell->zvals_sct_r);CHKERRQ(ierr);
    ierr = ISDestroy(&shell->zrows);CHKERRQ(ierr);
    ierr = ISDestroy(&shell->zcols);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode MatMissingDiagonal_Shell(Mat A,PetscBool  *missing,PetscInt *d)
{
  PetscFunctionBegin;
  *missing = PETSC_FALSE;
  PetscFunctionReturn(0);
}

PetscErrorCode MatAXPY_Shell(Mat Y,PetscScalar a,Mat X,MatStructure str)
{
  Mat_Shell      *shell = (Mat_Shell*)Y->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = PetscObjectReference((PetscObject)X);CHKERRQ(ierr);
  ierr = MatDestroy(&shell->axpy);CHKERRQ(ierr);
  shell->axpy        = X;
  shell->axpy_vscale = a;
  PetscFunctionReturn(0);
}

static struct _MatOps MatOps_Values = {0,
                                       0,
                                       0,
                                       0,
                                /* 4*/ MatMultAdd_Shell,
                                       0,
                                       MatMultTransposeAdd_Shell,
                                       0,
                                       0,
                                       0,
                                /*10*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*15*/ 0,
                                       0,
                                       0,
                                       MatDiagonalScale_Shell,
                                       0,
                                /*20*/ 0,
                                       MatAssemblyEnd_Shell,
                                       0,
                                       0,
                                /*24*/ MatZeroRows_Shell,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*29*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*34*/ MatDuplicate_Shell,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*39*/ MatAXPY_Shell,
                                       0,
                                       0,
                                       0,
                                       MatCopy_Shell,
                                /*44*/ 0,
                                       MatScale_Shell,
                                       MatShift_Shell,
                                       MatDiagonalSet_Shell,
                                       MatZeroRowsColumns_Shell,
                                /*49*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*54*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*59*/ 0,
                                       MatDestroy_Shell,
                                       0,
                                       MatConvertFrom_Shell,
                                       0,
                                /*64*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*69*/ 0,
                                       0,
                                       MatConvert_Shell,
                                       0,
                                       0,
                                /*74*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*79*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*84*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*89*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*94*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                                /*99*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                               /*104*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                               /*109*/ 0,
                                       0,
                                       0,
                                       0,
                                       MatMissingDiagonal_Shell,
                               /*114*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                               /*119*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                               /*124*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                               /*129*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                               /*134*/ 0,
                                       0,
                                       0,
                                       0,
                                       0,
                               /*139*/ 0,
                                       0,
                                       0
};

PetscErrorCode  MatShellSetContext_Shell(Mat mat,void *ctx)
{
  Mat_Shell      *shell = (Mat_Shell*)mat->data;

  PetscFunctionBegin;
  shell->ctx = ctx;
  PetscFunctionReturn(0);
}

static PetscErrorCode MatShellSetVecType_Shell(Mat mat,VecType vtype)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = PetscFree(mat->defaultvectype);CHKERRQ(ierr);
  ierr = PetscStrallocpy(vtype,(char**)&mat->defaultvectype);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

PetscErrorCode MatShellSetManageScalingShifts_Shell(Mat A)
{
  Mat_Shell      *shell = (Mat_Shell*)A->data;

  PetscFunctionBegin;
  shell->managescalingshifts = PETSC_FALSE;
  A->ops->diagonalset   = NULL;
  A->ops->diagonalscale = NULL;
  A->ops->scale         = NULL;
  A->ops->shift         = NULL;
  A->ops->axpy          = NULL;
  PetscFunctionReturn(0);
}

PetscErrorCode MatShellSetOperation_Shell(Mat mat,MatOperation op,void (*f)(void))
{
  Mat_Shell      *shell = (Mat_Shell*)mat->data;

  PetscFunctionBegin;
  switch (op) {
  case MATOP_DESTROY:
    shell->ops->destroy = (PetscErrorCode (*)(Mat))f;
    break;
  case MATOP_VIEW:
    if (!mat->ops->viewnative) {
      mat->ops->viewnative = mat->ops->view;
    }
    mat->ops->view = (PetscErrorCode (*)(Mat,PetscViewer))f;
    break;
  case MATOP_COPY:
    shell->ops->copy = (PetscErrorCode (*)(Mat,Mat,MatStructure))f;
    break;
  case MATOP_DIAGONAL_SET:
  case MATOP_DIAGONAL_SCALE:
  case MATOP_SHIFT:
  case MATOP_SCALE:
  case MATOP_AXPY:
  case MATOP_ZERO_ROWS:
  case MATOP_ZERO_ROWS_COLUMNS:
    if (shell->managescalingshifts) SETERRQ(PetscObjectComm((PetscObject)mat),PETSC_ERR_ARG_WRONGSTATE,"MATSHELL is managing scalings and shifts, see MatShellSetManageScalingShifts()");
    (((void(**)(void))mat->ops)[op]) = f;
    break;
  case MATOP_GET_DIAGONAL:
    if (shell->managescalingshifts) {
      shell->ops->getdiagonal = (PetscErrorCode (*)(Mat,Vec))f;
      mat->ops->getdiagonal   = MatGetDiagonal_Shell;
    } else {
      shell->ops->getdiagonal = NULL;
      mat->ops->getdiagonal   = (PetscErrorCode (*)(Mat,Vec))f;
    }
    break;
  case MATOP_MULT:
    if (shell->managescalingshifts) {
      shell->ops->mult = (PetscErrorCode (*)(Mat,Vec,Vec))f;
      mat->ops->mult   = MatMult_Shell;
    } else {
      shell->ops->mult = NULL;
      mat->ops->mult   = (PetscErrorCode (*)(Mat,Vec,Vec))f;
    }
    break;
  case MATOP_MULT_TRANSPOSE:
    if (shell->managescalingshifts) {
      shell->ops->multtranspose = (PetscErrorCode (*)(Mat,Vec,Vec))f;
      mat->ops->multtranspose   = MatMultTranspose_Shell;
    } else {
      shell->ops->multtranspose = NULL;
      mat->ops->multtranspose   = (PetscErrorCode (*)(Mat,Vec,Vec))f;
    }
    break;
  default:
    (((void(**)(void))mat->ops)[op]) = f;
    break;
  }
  PetscFunctionReturn(0);
}

PetscErrorCode MatShellGetOperation_Shell(Mat mat,MatOperation op,void(**f)(void))
{
  Mat_Shell      *shell = (Mat_Shell*)mat->data;

  PetscFunctionBegin;
  switch (op) {
  case MATOP_DESTROY:
    *f = (void (*)(void))shell->ops->destroy;
    break;
  case MATOP_VIEW:
    *f = (void (*)(void))mat->ops->view;
    break;
  case MATOP_COPY:
    *f = (void (*)(void))shell->ops->copy;
    break;
  case MATOP_DIAGONAL_SET:
  case MATOP_DIAGONAL_SCALE:
  case MATOP_SHIFT:
  case MATOP_SCALE:
  case MATOP_AXPY:
  case MATOP_ZERO_ROWS:
  case MATOP_ZERO_ROWS_COLUMNS:
    *f = (((void (**)(void))mat->ops)[op]);
    break;
  case MATOP_GET_DIAGONAL:
    if (shell->ops->getdiagonal)
      *f = (void (*)(void))shell->ops->getdiagonal;
    else
      *f = (((void (**)(void))mat->ops)[op]);
    break;
  case MATOP_MULT:
    if (shell->ops->mult)
      *f = (void (*)(void))shell->ops->mult;
    else
      *f = (((void (**)(void))mat->ops)[op]);
    break;
  case MATOP_MULT_TRANSPOSE:
    if (shell->ops->multtranspose)
      *f = (void (*)(void))shell->ops->multtranspose;
    else
      *f = (((void (**)(void))mat->ops)[op]);
    break;
  default:
    *f = (((void (**)(void))mat->ops)[op]);
  }
  PetscFunctionReturn(0);
}

/*MC
   MATSHELL - MATSHELL = "shell" - A matrix type to be used to define your own matrix type -- perhaps matrix free.

  Level: advanced

.seealso: MatCreateShell()
M*/

PETSC_EXTERN PetscErrorCode MatCreate_Shell(Mat A)
{
  Mat_Shell      *b;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = PetscMemcpy(A->ops,&MatOps_Values,sizeof(struct _MatOps));CHKERRQ(ierr);

  ierr    = PetscNewLog(A,&b);CHKERRQ(ierr);
  A->data = (void*)b;

  ierr = PetscLayoutSetUp(A->rmap);CHKERRQ(ierr);
  ierr = PetscLayoutSetUp(A->cmap);CHKERRQ(ierr);

  b->ctx                 = 0;
  b->vshift              = 0.0;
  b->vscale              = 1.0;
  b->managescalingshifts = PETSC_TRUE;
  A->assembled           = PETSC_TRUE;
  A->preallocated        = PETSC_FALSE;

  ierr = PetscObjectComposeFunction((PetscObject)A,"MatShellGetContext_C",MatShellGetContext_Shell);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)A,"MatShellSetContext_C",MatShellSetContext_Shell);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)A,"MatShellSetVecType_C",MatShellSetVecType_Shell);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)A,"MatShellSetManageScalingShifts_C",MatShellSetManageScalingShifts_Shell);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)A,"MatShellSetOperation_C",MatShellSetOperation_Shell);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)A,"MatShellGetOperation_C",MatShellGetOperation_Shell);CHKERRQ(ierr);
  ierr = PetscObjectChangeTypeName((PetscObject)A,MATSHELL);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*@C
   MatCreateShell - Creates a new matrix class for use with a user-defined
   private data storage format.

  Collective

   Input Parameters:
+  comm - MPI communicator
.  m - number of local rows (must be given)
.  n - number of local columns (must be given)
.  M - number of global rows (may be PETSC_DETERMINE)
.  N - number of global columns (may be PETSC_DETERMINE)
-  ctx - pointer to data needed by the shell matrix routines

   Output Parameter:
.  A - the matrix

   Level: advanced

  Usage:
$    extern PetscErrorCode mult(Mat,Vec,Vec);
$    MatCreateShell(comm,m,n,M,N,ctx,&mat);
$    MatShellSetOperation(mat,MATOP_MULT,(void(*)(void))mult);
$    [ Use matrix for operations that have been set ]
$    MatDestroy(mat);

   Notes:
   The shell matrix type is intended to provide a simple class to use
   with KSP (such as, for use with matrix-free methods). You should not
   use the shell type if you plan to define a complete matrix class.

   Fortran Notes:
    To use this from Fortran with a ctx you must write an interface definition for this
    function and for MatShellGetContext() that tells Fortran the Fortran derived data type you are passing
    in as the ctx argument.

   PETSc requires that matrices and vectors being used for certain
   operations are partitioned accordingly.  For example, when
   creating a shell matrix, A, that supports parallel matrix-vector
   products using MatMult(A,x,y) the user should set the number
   of local matrix rows to be the number of local elements of the
   corresponding result vector, y. Note that this is information is
   required for use of the matrix interface routines, even though
   the shell matrix may not actually be physically partitioned.
   For example,

$
$     Vec x, y
$     extern PetscErrorCode mult(Mat,Vec,Vec);
$     Mat A
$
$     VecCreateMPI(comm,PETSC_DECIDE,M,&y);
$     VecCreateMPI(comm,PETSC_DECIDE,N,&x);
$     VecGetLocalSize(y,&m);
$     VecGetLocalSize(x,&n);
$     MatCreateShell(comm,m,n,M,N,ctx,&A);
$     MatShellSetOperation(mat,MATOP_MULT,(void(*)(void))mult);
$     MatMult(A,x,y);
$     MatDestroy(&A);
$     VecDestroy(&y);
$     VecDestroy(&x);
$


   MATSHELL handles MatShift(), MatDiagonalSet(), MatDiagonalScale(), MatAXPY(), MatScale(), MatZeroRows() and MatZeroRowsColumns() internally, so these
   operations cannot be overwritten unless MatShellSetManageScalingShifts() is called.


    For rectangular matrices do all the scalings and shifts make sense?

    Developers Notes:
    Regarding shifting and scaling. The general form is

          diag(left)(vscale*A + diag(dshift) + vshift I)diag(right)

      The order you apply the operations is important. For example if you have a dshift then
      apply a MatScale(s) you get s*vscale*A + s*diag(shift). But if you first scale and then shift
      you get s*vscale*A + diag(shift)

          A is the user provided function.

   KSP/PC uses changes in the Mat's "state" to decide if preconditioners need to be rebuilt: PCSetUp() only calls the setup() for
   for the PC implementation if the Mat state has increased from the previous call. Thus to get changes in a MATSHELL to trigger
   an update in the preconditioner you must call MatAssemblyBegin()/MatAssemblyEnd() or PetscObjectStateIncrease((PetscObject)mat);
   each time the MATSHELL matrix has changed.

   Calling MatAssemblyBegin()/MatAssemblyEnd() on a MATSHELL removes any previously supplied shift and scales that were provided
   with MatDiagonalSet(), MatShift(), MatScale(), or MatDiagonalScale().

.seealso: MatShellSetOperation(), MatHasOperation(), MatShellGetContext(), MatShellSetContext(), MATSHELL, MatShellSetManageScalingShifts()
@*/
PetscErrorCode  MatCreateShell(MPI_Comm comm,PetscInt m,PetscInt n,PetscInt M,PetscInt N,void *ctx,Mat *A)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = MatCreate(comm,A);CHKERRQ(ierr);
  ierr = MatSetSizes(*A,m,n,M,N);CHKERRQ(ierr);
  ierr = MatSetType(*A,MATSHELL);CHKERRQ(ierr);
  ierr = MatShellSetContext(*A,ctx);CHKERRQ(ierr);
  ierr = MatSetUp(*A);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}


/*@
    MatShellSetContext - sets the context for a shell matrix

   Logically Collective on Mat

    Input Parameters:
+   mat - the shell matrix
-   ctx - the context

   Level: advanced

   Fortran Notes:
    To use this from Fortran you must write a Fortran interface definition for this
    function that tells Fortran the Fortran derived data type that you are passing in as the ctx argument.

.seealso: MatCreateShell(), MatShellGetContext(), MatShellGetOperation()
@*/
PetscErrorCode  MatShellSetContext(Mat mat,void *ctx)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(mat,MAT_CLASSID,1);
  ierr = PetscUseMethod(mat,"MatShellSetContext_C",(Mat,void*),(mat,ctx));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*@C
 MatShellSetVecType - Sets the type of Vec returned by MatCreateVecs()

 Logically collective

    Input Parameters:
+   mat   - the shell matrix
-   vtype - type to use for creating vectors

 Notes:

 Level: advanced

.seealso: MatCreateVecs()
@*/
PetscErrorCode  MatShellSetVecType(Mat mat,VecType vtype)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = PetscTryMethod(mat,"MatShellSetVecType_C",(Mat,VecType),(mat,vtype));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*@
    MatShellSetManageScalingShifts - Allows the user to control the scaling and shift operations of the MATSHELL. Must be called immediately
          after MatCreateShell()

   Logically Collective on Mat

    Input Parameter:
.   mat - the shell matrix

  Level: advanced

.seealso: MatCreateShell(), MatShellGetContext(), MatShellGetOperation(), MatShellSetContext(), MatShellSetOperation()
@*/
PetscErrorCode MatShellSetManageScalingShifts(Mat A)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(A,MAT_CLASSID,1);
  ierr = PetscUseMethod(A,"MatShellSetManageScalingShifts_C",(Mat),(A));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*@C
    MatShellTestMult - Compares the multiply routine provided to the MATSHELL with differencing on a given function.

   Logically Collective on Mat

    Input Parameters:
+   mat - the shell matrix
.   f - the function
.   base - differences are computed around this vector, see MatMFFDSetBase(), for Jacobians this is the point at which the Jacobian is being evaluated
-   ctx - an optional context for the function

   Output Parameter:
.   flg - PETSC_TRUE if the multiply is likely correct

   Options Database:
.   -mat_shell_test_mult_view - print if any differences are detected between the products and print the difference

   Level: advanced

   Fortran Notes:
    Not supported from Fortran

.seealso: MatCreateShell(), MatShellGetContext(), MatShellGetOperation(), MatShellTestMultTranspose()
@*/
PetscErrorCode  MatShellTestMult(Mat mat,PetscErrorCode (*f)(void*,Vec,Vec),Vec base,void *ctx,PetscBool *flg)
{
  PetscErrorCode ierr;
  PetscInt       m,n;
  Mat            mf,Dmf,Dmat,Ddiff;
  PetscReal      Diffnorm,Dmfnorm;
  PetscBool      v = PETSC_FALSE, flag = PETSC_TRUE;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(mat,MAT_CLASSID,1);
  ierr = PetscOptionsHasName(NULL,((PetscObject)mat)->prefix,"-mat_shell_test_mult_view",&v);CHKERRQ(ierr);
  ierr = MatGetLocalSize(mat,&m,&n);CHKERRQ(ierr);
  ierr = MatCreateMFFD(PetscObjectComm((PetscObject)mat),m,n,PETSC_DECIDE,PETSC_DECIDE,&mf);CHKERRQ(ierr);
  ierr = MatMFFDSetFunction(mf,f,ctx);CHKERRQ(ierr);
  ierr = MatMFFDSetBase(mf,base,NULL);CHKERRQ(ierr);

  ierr = MatComputeOperator(mf,MATAIJ,&Dmf);CHKERRQ(ierr);
  ierr = MatComputeOperator(mat,MATAIJ,&Dmat);CHKERRQ(ierr);

  ierr = MatDuplicate(Dmat,MAT_COPY_VALUES,&Ddiff);CHKERRQ(ierr);
  ierr = MatAXPY(Ddiff,-1.0,Dmf,DIFFERENT_NONZERO_PATTERN);CHKERRQ(ierr);
  ierr = MatNorm(Ddiff,NORM_FROBENIUS,&Diffnorm);CHKERRQ(ierr);
  ierr = MatNorm(Dmf,NORM_FROBENIUS,&Dmfnorm);CHKERRQ(ierr);
  if (Diffnorm/Dmfnorm > 10*PETSC_SQRT_MACHINE_EPSILON) {
    flag = PETSC_FALSE;
    if (v) {
      ierr = PetscPrintf(PetscObjectComm((PetscObject)mat),"MATSHELL and matrix free multiple appear to produce different results.\n Norm Ratio %g Difference results followed by finite difference one\n",(double)(Diffnorm/Dmfnorm));CHKERRQ(ierr);
      ierr = MatViewFromOptions(Ddiff,(PetscObject)mat,"-mat_shell_test_mult_view");CHKERRQ(ierr);
      ierr = MatViewFromOptions(Dmf,(PetscObject)mat,"-mat_shell_test_mult_view");CHKERRQ(ierr);
      ierr = MatViewFromOptions(Dmat,(PetscObject)mat,"-mat_shell_test_mult_view");CHKERRQ(ierr);
    }
  } else if (v) {
    ierr = PetscPrintf(PetscObjectComm((PetscObject)mat),"MATSHELL and matrix free multiple appear to produce the same results\n");CHKERRQ(ierr);
  }
  if (flg) *flg = flag;
  ierr = MatDestroy(&Ddiff);CHKERRQ(ierr);
  ierr = MatDestroy(&mf);CHKERRQ(ierr);
  ierr = MatDestroy(&Dmf);CHKERRQ(ierr);
  ierr = MatDestroy(&Dmat);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*@C
    MatShellTestMultTranpose - Compares the multiply transpose routine provided to the MATSHELL with differencing on a given function.

   Logically Collective on Mat

    Input Parameters:
+   mat - the shell matrix
.   f - the function
.   base - differences are computed around this vector, see MatMFFDSetBase(), for Jacobians this is the point at which the Jacobian is being evaluated
-   ctx - an optional context for the function

   Output Parameter:
.   flg - PETSC_TRUE if the multiply is likely correct

   Options Database:
.   -mat_shell_test_mult_view - print if any differences are detected between the products and print the difference

   Level: advanced

   Fortran Notes:
    Not supported from Fortran

.seealso: MatCreateShell(), MatShellGetContext(), MatShellGetOperation(), MatShellTestMult()
@*/
PetscErrorCode  MatShellTestMultTranspose(Mat mat,PetscErrorCode (*f)(void*,Vec,Vec),Vec base,void *ctx,PetscBool *flg)
{
  PetscErrorCode ierr;
  Vec            x,y,z;
  PetscInt       m,n,M,N;
  Mat            mf,Dmf,Dmat,Ddiff;
  PetscReal      Diffnorm,Dmfnorm;
  PetscBool      v = PETSC_FALSE, flag = PETSC_TRUE;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(mat,MAT_CLASSID,1);
  ierr = PetscOptionsHasName(NULL,((PetscObject)mat)->prefix,"-mat_shell_test_mult_transpose_view",&v);CHKERRQ(ierr);
  ierr = MatCreateVecs(mat,&x,&y);CHKERRQ(ierr);
  ierr = VecDuplicate(y,&z);CHKERRQ(ierr);
  ierr = MatGetLocalSize(mat,&m,&n);CHKERRQ(ierr);
  ierr = MatGetSize(mat,&M,&N);CHKERRQ(ierr);
  ierr = MatCreateMFFD(PetscObjectComm((PetscObject)mat),m,n,M,N,&mf);CHKERRQ(ierr);
  ierr = MatMFFDSetFunction(mf,f,ctx);CHKERRQ(ierr);
  ierr = MatMFFDSetBase(mf,base,NULL);CHKERRQ(ierr);
  ierr = MatComputeOperator(mf,MATAIJ,&Dmf);CHKERRQ(ierr);
  ierr = MatTranspose(Dmf,MAT_INPLACE_MATRIX,&Dmf);CHKERRQ(ierr);
  ierr = MatComputeOperatorTranspose(mat,MATAIJ,&Dmat);CHKERRQ(ierr);

  ierr = MatDuplicate(Dmat,MAT_COPY_VALUES,&Ddiff);CHKERRQ(ierr);
  ierr = MatAXPY(Ddiff,-1.0,Dmf,DIFFERENT_NONZERO_PATTERN);CHKERRQ(ierr);
  ierr = MatNorm(Ddiff,NORM_FROBENIUS,&Diffnorm);CHKERRQ(ierr);
  ierr = MatNorm(Dmf,NORM_FROBENIUS,&Dmfnorm);CHKERRQ(ierr);
  if (Diffnorm/Dmfnorm > 10*PETSC_SQRT_MACHINE_EPSILON) {
    flag = PETSC_FALSE;
    if (v) {
      ierr = PetscPrintf(PetscObjectComm((PetscObject)mat),"MATSHELL and matrix free multiple appear to produce different results.\n Norm Ratio %g Difference results followed by finite difference one\n",(double)(Diffnorm/Dmfnorm));CHKERRQ(ierr);
      ierr = MatViewFromOptions(Ddiff,(PetscObject)mat,"-mat_shell_test_mult_transpose_view");CHKERRQ(ierr);
      ierr = MatViewFromOptions(Dmf,(PetscObject)mat,"-mat_shell_test_mult_transpose_view");CHKERRQ(ierr);
      ierr = MatViewFromOptions(Dmat,(PetscObject)mat,"-mat_shell_test_mult_transpose_view");CHKERRQ(ierr);
    }
  } else if (v) {
    ierr = PetscPrintf(PetscObjectComm((PetscObject)mat),"MATSHELL transpose and matrix free multiple appear to produce the same results\n");CHKERRQ(ierr);
  }
  if (flg) *flg = flag;
  ierr = MatDestroy(&mf);CHKERRQ(ierr);
  ierr = MatDestroy(&Dmat);CHKERRQ(ierr);
  ierr = MatDestroy(&Ddiff);CHKERRQ(ierr);
  ierr = MatDestroy(&Dmf);CHKERRQ(ierr);
  ierr = VecDestroy(&x);CHKERRQ(ierr);
  ierr = VecDestroy(&y);CHKERRQ(ierr);
  ierr = VecDestroy(&z);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*@C
    MatShellSetOperation - Allows user to set a matrix operation for a shell matrix.

   Logically Collective on Mat

    Input Parameters:
+   mat - the shell matrix
.   op - the name of the operation
-   g - the function that provides the operation.

   Level: advanced

    Usage:
$      extern PetscErrorCode usermult(Mat,Vec,Vec);
$      ierr = MatCreateShell(comm,m,n,M,N,ctx,&A);
$      ierr = MatShellSetOperation(A,MATOP_MULT,(void(*)(void))usermult);

    Notes:
    See the file include/petscmat.h for a complete list of matrix
    operations, which all have the form MATOP_<OPERATION>, where
    <OPERATION> is the name (in all capital letters) of the
    user interface routine (e.g., MatMult() -> MATOP_MULT).

    All user-provided functions (except for MATOP_DESTROY) should have the same calling
    sequence as the usual matrix interface routines, since they
    are intended to be accessed via the usual matrix interface
    routines, e.g.,
$       MatMult(Mat,Vec,Vec) -> usermult(Mat,Vec,Vec)

    In particular each function MUST return an error code of 0 on success and
    nonzero on failure.

    Within each user-defined routine, the user should call
    MatShellGetContext() to obtain the user-defined context that was
    set by MatCreateShell().

    Fortran Notes:
    For MatCreateVecs() the user code should check if the input left or right matrix is -1 and in that case not
       generate a matrix. See src/mat/examples/tests/ex120f.F

    Use MatSetOperation() to set an operation for any matrix type

.seealso: MatCreateShell(), MatShellGetContext(), MatShellGetOperation(), MatShellSetContext(), MatSetOperation(), MatShellSetManageScalingShifts()
@*/
PetscErrorCode MatShellSetOperation(Mat mat,MatOperation op,void (*g)(void))
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(mat,MAT_CLASSID,1);
  ierr = PetscUseMethod(mat,"MatShellSetOperation_C",(Mat,MatOperation,void (*)(void)),(mat,op,g));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*@C
    MatShellGetOperation - Gets a matrix function for a shell matrix.

    Not Collective

    Input Parameters:
+   mat - the shell matrix
-   op - the name of the operation

    Output Parameter:
.   g - the function that provides the operation.

    Level: advanced

    Notes:
    See the file include/petscmat.h for a complete list of matrix
    operations, which all have the form MATOP_<OPERATION>, where
    <OPERATION> is the name (in all capital letters) of the
    user interface routine (e.g., MatMult() -> MATOP_MULT).

    All user-provided functions have the same calling
    sequence as the usual matrix interface routines, since they
    are intended to be accessed via the usual matrix interface
    routines, e.g.,
$       MatMult(Mat,Vec,Vec) -> usermult(Mat,Vec,Vec)

    Within each user-defined routine, the user should call
    MatShellGetContext() to obtain the user-defined context that was
    set by MatCreateShell().

.seealso: MatCreateShell(), MatShellGetContext(), MatShellSetOperation(), MatShellSetContext()
@*/
PetscErrorCode MatShellGetOperation(Mat mat,MatOperation op,void(**g)(void))
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(mat,MAT_CLASSID,1);
  ierr = PetscUseMethod(mat,"MatShellGetOperation_C",(Mat,MatOperation,void (**)(void)),(mat,op,g));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}