+extendSubstList :: Subst -> [(Var,CoreArg)] -> Subst
+extendSubstList subst [] = subst
+extendSubstList subst ((var,rhs):prs) = extendSubstList (extendSubst subst var rhs) prs
+
+extendSubst :: Subst -> Var -> CoreArg -> Subst
+extendSubst (Subst in_scope ids tvs) tv (Type ty)
+ = ASSERT( isTyVar tv ) Subst in_scope ids (extendVarEnv tvs tv ty)
+extendSubst (Subst in_scope ids tvs) id expr
+ = ASSERT( isId id ) Subst in_scope (extendVarEnv ids id expr) tvs
+