proposition: Lemma size_subseq s1 s2 : subseq s1 s2 -> size s1 <= size s2.
proof: Proof. by case/subseqP=> m sz_m ->; rewrite size_mask -sz_m ?count_size. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)