proposition: Lemma behead_bseqP s : size (behead s) <= n.-1.
proof: Proof. rewrite size_behead -!subn1; apply/leq_sub2r/size_bseq. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq choice fintype path)