proposition: Lemma rcons2_infix s1 s2 x : infix (rcons s1 x) (rcons s2 x) -> infix s1 s2.
proof: Proof. by rewrite -infix_rev !rev_rcons => /cons2_infix; rewrite infix_rev. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)