(* * PCP to CFPP *)
Hint Rewrite concat_app map_app map_map : list.
Hint Rewrite <- map_rev : list.
Definition gamma (A : stack nat) := map (fun '(x,y) => (x, rev y)) A.

Lemma sigma_inv A s1 s x y :
sigma s1 A = x ++ [s] ++ y -> ~ s el x -> ~ s el y -> ~ s el sym A ->
s1 = s.