But thinking about this today, I have no idea what MPI_REPLACE is
supposed to do in a collective reduction. Specifically -- what value
should end up in the target buffer? It doesn't make sense.
I think that this is a grey area in the MPI standard -- MPI_REPLACE is
an MPI_Op, but it should *only
Brian and I just noticed something -- a long time ago, I thought I was
being clever and forward-thinking by ensuring to define MPI_REPLACE as
a valid reduction operator for MPI_REDUCE (and friends). The careful
reader will remember that MPI_REPLACE is the MPI-2 MPI_Op added for
MPI_ACCUMULATE
