+-- | Add a substitution for a 'TyVar' or 'Id' as appropriate to the 'Var' being added. See also
+-- 'extendIdSubst' and 'extendTvSubst'
+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
+
+-- | Add a substitution for a 'TyVar' or 'Id' as appropriate to all the 'Var's being added. See also 'extendSubst'
+extendSubstList :: Subst -> [(Var,CoreArg)] -> Subst
+extendSubstList subst [] = subst
+extendSubstList subst ((var,rhs):prs) = extendSubstList (extendSubst subst var rhs) prs
+
+-- | Find the substitution for an 'Id' in the 'Subst'