proposition: Lemma in_setD1 x A b : (x \in A :\ b) = (x != b) && (x \in A) .
proof: Proof. by rewrite !inE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)