Definition palin X (A : list X) := A = rev A.

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