we need to have a meeting to discuss how to implement handles and predefined handle constants e.g. MPI_COMM_WORLD and MPI_INT.  i think we have consensus that we will do e.g. typedef struct MPI_ABI_Datatype * MPI_Datatype;  since that is the only option that provides type-safety.  there is an open question about using integer literal constants (MPICH-style) or addresses (OMPI-style).  there are merits to both.