return MPI_ERR_COMM;
//non commutative case, use a working algo from openmpi
if (op != MPI_OP_NULL && not op->is_commutative()) {
return MPI_ERR_COMM;
//non commutative case, use a working algo from openmpi
if (op != MPI_OP_NULL && not op->is_commutative()) {