proposition: Lemma setU1P x a B : reflect (x = a \/ x \in B) (x \in a |: B).
proof: Proof. by rewrite !inE; apply: predU1P. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)