typedef struct _PetscFormKey { DMLabel label; /* The (label, value) select a subdomain */ PetscInt value; PetscInt field; /* Selects the field for the test function */ PetscInt part; /* Selects the equation part. For example, LHS = 0 and RHS = 1 in IMEX methods. More pieces can be present for operator splitting methods. */ } PetscFormKey;The subdomain on which to integrate is specified by (label, value), the test function field by (field), and the piece of the equation by (part). For example, LHS = 0 and RHS = 1 in IMEX methods. More pieces can be present for operator splitting methods.