proposition: Lemma exprAC x m n : (x ^+ m) ^+ n = (x ^+ n) ^+ m.
proof: Proof. by rewrite -!exprM mulnC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)