proposition: Lemma rpred_divl x y : x \in S -> x \is a unit -> (x / y \in S) = (y \in S).
proof: Proof. by rewrite -(rpredV y); apply: rpredMl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)