-Note [Rule dependency info]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
-THe VarSet in a SpecInfo is used for dependency analysis in the
-occurrence analyser. We must track free vars in *both* lhs and rhs. Why both?
-Consider
- x = y
- RULE f x = 4
-Then if we substitute y for x, we'd better do so in the
- rule's LHS too, so we'd better ensure the dependency is respsected
-