match.arg()
that uses comm.stop()
rather
so that the error message (if there is one) is managed according
to the rules of .pbd_env$SPMD.CT
.comm.match.arg(arg, choices, several.ok=FALSE, ...,
all.rank = .pbd_env$SPMD.CT$print.all.rank,
rank.print = .pbd_env$SPMD.CT$rank.source,
comm = .pbd_env$SPMD.CT$comm,
mpi.finalize = .pbd_env$SPMD.CT$mpi.finalize,
quit = .pbd_env$SPMD.CT$quit)