Section CFGs.
Global Instance rewtTrans R :
PreOrder (rewt R).
Global Instance rewrite_proper R :
Proper (rewt R ==> rewt R ==> rewt R) (@app sig).
Global Instance subrel R :
subrelation (rew_cfg R) (rewt R).

Lemma terminal_iff G x :
terminal G x <-> forall s y, s el x -> ~ (s, y) el rules G.