/usr/bin/mb-mpi is in mrbayes-mpi 3.1.2-2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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#!/bin/sh # PATH="/usr/lib/mrbayes-mpi:$PATH" orterun /usr/lib/mrbayes-mpi/mb-mpi $@