There is also a Req argument to mrgsim that can
be set to accomplish the same thing as a call to Req in
the pipeline.
Note the difference between req and Req: the former only
selects compartments to appear in output while the latter selects both
compartments and captured items. Also, when there are items are explicitly
listed in Req, all other compartments or captured items not listed
there are ignored. But when compartments are selected with req
all of the captured items are returned. Remember that req is
strictly for compartments.