proposition: Lemma lincl_refl : forall (l:seq T), lincl l l.
proof: Proof. elim => [ | hd tl hi] //=. apply/andP; split. - by rewrite in_cons eq_refl. - by apply: lincl_cons. Qed.
imports: Coq(Mergesort Permutation) HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq fintype) mathcomp(path)