proposition: Lemma expfB_cond m n x : (x == 0) + n <= m -> x ^+ (m - n) = x ^+ m / x ^+ n.
proof: Proof. move/subnK=> <-; rewrite addnA addnK !exprD. have [-> | nz_x] := eqVneq; first by rewrite !mulr0 !mul0r. by rewrite mulfK ?expf_neq0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)