proposition: Lemma prl_r3_nI_stable_union x y: inc x prl_r3_nI -> inc y prl_r3_nI -> inc (x \cup y) prl_r3_nI.
proof: Proof. move: (finite_subsets_order (psI S)) => [_ h _ _ _]; apply: h. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)