 or not" Can this also be
done with communicators instead of groups? In principle yes, since the
comm field will have a MPI<sub>COMM</sub><sub>NULL</sub> value if the process does not
belong.  However, those processors not in a given communicator will
not know the corresponding group number, so the ranks in World will
not be available for the kinds of "inter-communication" that we are
doing.
</p>

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<pre class="src src-f90"><span style="color: #a020f0;">subroutine</span> <span