(* * 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_eq  s A :
sigma s A = tau1 A ++ [s] ++ rev (tau2 (gamma A)).