proposition: Lemma set2P x a b : reflect (x = a \/ x = b) (x \in [set a; b]).
proof: Proof. by rewrite !inE; apply: pred2P. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)