-- Is this defined elsewhere?
-- Generalize the type: find all free tyvars and wrap in the appropiate ForAll.
sigmaType :: Type -> Type
-sigmaType ty = mkForAllTys (varSetElems$ tyVarsOfType (dropForAlls ty)) ty
+sigmaType ty = mkSigmaTy (varSetElems$ tyVarsOfType (dropForAlls ty)) [] ty