proposition: Lemma setU1_P A b z: inc z (A +s1 b) <-> inc z A \/ z = b.
proof: Proof. split; first by case /setU2_P; [left | move /set1_P; right]. by case =>h; apply/setU2_P; [by left | right; apply/set1_P]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)