proposition: Lemma num_occ_rev (t : seq A) : N(a | t) = N(a | rev t).
proof: Proof. elim : t => // hd tl IH /=. by rewrite IH rev_cons /= {2}/num_occ -cats1 count_cat /= addn0 addnC. Qed.
imports: mathcomp(all_ssreflect fingroup perm zmodp ssralg) ssr_ext f2