proposition: Lemma succ_psi a b: [a, b] != one -> T2succ [a,b] = cons a b 0 one.
proof: Proof. by rewrite /= T2eqE !eqxx !andbT; case (_ && _). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)