proposition: Lemma catr_infix s s1 s2 : infix (s1 ++ s) s2 -> infix s1 s2.
proof: Proof. by rewrite -infix_rev rev_cat => /catl_infix; rewrite infix_rev. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)