proposition: Lemma subrel_trans : subrel r1 r2 -> subrel r2 r3 -> subrel r1 r3.
proof: Proof. move=> Hsub1 Hsub2 a b Hr1. by apply Hsub2, Hsub1. Qed.
imports: mathcomp(all_ssreflect ssralg fingroup zmodp poly ssrnum) mathcomp(matrix perm) ssr_ext ssralg_ext fdist