+ ---------------------------------------------
+ The inner workings of matching
+ ---------------------------------------------
+
+\begin{code}
+-- These two definitions are not the same as in Subst,
+-- but they simple and direct, and purely local to this module
+-- The third, for TvSubstEnv, is the same as in VarEnv, but repeated here
+-- for uniformity with IdSubstEnv
+type SubstEnv = (TvSubstEnv, IdSubstEnv)
+type IdSubstEnv = IdEnv CoreExpr
+type TvSubstEnv = TyVarEnv Type
+
+emptySubstEnv :: SubstEnv