Definition PALIN (G : cfg) := exists x, L G x /\ x = List.rev x.Definition Sigma := sym P.Definition inter_nonempty '(G1, G2) :=
    exists x, @L G1 x /\ L G2 x.

(* Specifications: *)
Lemma terminal_G1 x : ~ Start el x <-> terminal G1 x.
Lemma terminal_G2 x : ~ Start el x <-> terminal G2 x.