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.