I might be wrong, but is this order correct? Looking at the logic in lines 168-180 below, it seems this should be:
 1. If the -f argument is supplied, ROSE_COMMAND_FILE is that 
 2. If not, and there's a rose-mpi-launch.rc present, ROSE_COMMAND_FILE is set to $PWD/rose-mpi-launch.rc
 3. Otherwise, use set ROSE_COMMAND to be the first argument

This implies that the order of precedence is 2, 1, 3 in these comment. Or I'm misunderstanding something.