Move custom rf class outside the combsimp definition

This fixes, or rather, works around, a bizarre caching issue involving classes
defined inside functions.  See the discussion on pull request #1710.
change rf to _rf in simplify
Merge pull request #58 from smichr/rf

change rf to _rf in simplify
Merge pull request #1959 from asmeurer/rf

Move custom rf class outside the combsimp definition