proposition: Lemma lincl_catr : forall (l l1 l2: seq T), lincl l l1 -> lincl l (l1 ++ l2).
proof: Proof. elim => [ | hd tl hi] l1 l2 //=. case/andP => hhd htl. by rewrite mem_cat hhd; apply : hi. Qed.
imports: Coq(Mergesort Permutation) HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq fintype) mathcomp(path)