Deliberately not as a response to my other comment on this thread... The poster of
> http://stackoverflow.com/questions/19529607/how-to-pass-part-of-an-argument-to-a-gnu-parallel-command/19529783#19529783 seems to want to shoehorn nontrivial processing of arguments based on things on the RHS of :::. I think modifying his/her input using some other mechanism and then piping it into parallel seems more un*x-y and doesn't demand any $P-like machinery. - Rhys
