+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
+
+