typedef enum {TAO_SUBSET_SUBVEC,TAO_SUBSET_MASK,TAO_SUBSET_MATRIXFREE} TaoSubsetType;
| TAO_SUBSET_SUBVEC | - TAO uses PETSc's MatCreateSubMatrix and VecGetSubVector | |
| TAO_SUBSET_MASK | - Matrices are zeroed out corresponding to active set entries | |
| TAO_SUBSET_MATRIXFREE | - Same as TAO_SUBSET_MASK, but can be applied to matrix-free operators |
| -different_hessian | - TAO will use a copy of the hessian operator for masking. By default TAO will directly alter the hessian operator. |