proposition: Lemma size_nseqs n s : s \in nseqs n -> size s = n.
proof: Proof. elim: n s => /= [|n IH] s. by rewrite mem_seq1 => /eqP ->. move/allpairsP => [[x xs]] /= [Hx Hxs] -> /=. by rewrite IH. Qed.
imports: mathcomp(all_ssreflect ssralg fingroup zmodp poly ssrnum) mathcomp(matrix perm) ssr_ext ssralg_ext fdist