proposition: Lemma mx'_cast m n : 'I_n -> (m + n.-1)%N = (m + n).-1.
proof: Proof. by case=> j /ltn_predK <-; rewrite addnS. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg countalg finalg zmodp)