proposition: Lemma disjointU2_pr3 a b x y: y <> x -> (a +c b) =c ((a *s1 x) \cup (b *s1 y)).
proof: Proof. move=> yx. rewrite -(disjointU2_rw _ _ yx) double_cardinal. by apply: csum2_pr; apply: doubleton_fam_variant. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)