proposition: Lemma size_char_seq m (X: {set 'I_m}): size (char_seq X) = m.
proof: Proof. by rewrite size_map size_enum_ord. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)