proposition: Lemma num_co_occ_perm (a : A) (b : B) (s : 'S_n) : N( a , b | perm_tuple s ta , perm_tuple s tb ) = N( a , b | ta , tb ).
proof: Proof. by rewrite /num_co_occ -(@num_occ_perm _ n (zip_tuple ta tb) (a, b) s) -zip_perm_tuple. Qed.
imports: mathcomp(all_ssreflect fingroup perm zmodp ssralg) ssr_ext f2