proposition: Lemma unitfE x : (x \in unit) = (x != 0).
proof: Proof. by apply/idP/idP=> [/(memPn _)-> | /fieldP]; rewrite ?unitr0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)