proposition: Theorem setUf_A L f g: unionf (unionf L g) f = unionf L (fun l => unionf (g l) f).
proof: Proof. set_extens t; move /setUf_P=> [y yu tfy]; [ move: yu | move: tfy]; move/setUf_P => [z zsg ygz]; apply/setUf_P; exists z =>//; union_tac. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)