proposition: Lemma setD1P x A b : reflect (x != b /\ x \in A) (x \in A :\ b).
proof: Proof. by rewrite !inE; apply: andP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)