proposition: Lemma exprB m n x : n <= m -> x \is a unit -> x ^+ (m - n) = x ^+ m / x ^+ n.
proof: Proof. by move/subnK=> {2}<- Ux; rewrite exprD mulrK ?unitrX. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)