Scheme rewt_ind' := Induction for rewt Sort Prop.
Instance PreOrder_rewt R : PreOrder (rewt R).

Lemma rewt_app_L R x x' y : rewt R x x' -> rewt R (y ++ x) (y ++ x').
Instance Proper_rewt R : Proper (rewt R ==> rewt R ==> rewt R) (@app symbol).