proposition: Lemma setUt_constant (I:Set) (f:I ->Set) (x:I): constantp f -> uniont f = f x.
proof: Proof. move=> cf; set_extens t; first by move/setUt_P => [z]; rewrite (cf x z). by move=> tp; apply/setUt_P; exists x. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)