simple induction 1.
inversion 1.
inversion 3; auto.
Defined.
Hint Resolve lemma2'.

(* Specifications: *)
Theorem lemma2 :
forall (vs ws : list word) (a : letter), R a vs ws -> good vs -> good ws.
Theorem lemma2 :
forall (vs ws : list word) (a : letter), R a vs ws -> good vs -> good ws.