proposition: Lemma has_predU a1 a2 s : has (predU a1 a2) s = has a1 s || has a2 s.
proof: Proof. by elim: s => //= x s ->; rewrite -!orbA; do !bool_congr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)