Section PCP_CFPP.
Variable P : stack nat.
Definition Sigma := sym P.
Notation "#" := (fresh Sigma).
Definition gamma (A : stack nat) := map (fun '(x,y) => (x, rev y)) A.
Definition palin {X} (A : list X) := A = rev A.

Lemma tau_eq_iff A a :
~ a el sym A ->
tau1 A = tau2 A <-> palin (sigma a (gamma A)).