After a while of being annoyed by the quirks and limitations the otherwise very useful program "superpose" I decided to write my own version of this utility.
The result is called suppose. It is written in the NAB language and is compiled for several popular architectures.
It does everything that superpose does plus is very flexible in the way you define an arbitrary selection of atoms to be used for both fitting structures and/or computing RMSDs. It makes pretty nice, consistent, easy to read output and is well documented in its html formatted man page. The actual source for the real man page lives at /thr/frege/man/man1/suppose.1. You can either put /thr/frege/man in your MANPATH environment variable or copy this file to somewhere that is already in your MANPATH. Then "man suppose" should get you going.
Contact Jarrod Smith if you have bug reports, questions or suggestions regarding suppose.