proposition: Fact lex1 t : t <= [tuple of nseq n \top] :> n.-tuple T.
proof: Proof. by rewrite leEtprod; apply/forallP => i; rewrite tnth_nseq lex1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)