Definition setTfor (phT : phant T) := [set x : T | true].

Lemma mem_imset2 (D : pred aT) (D2 : aT -> pred aT2) x x2 :
x \in D -> x2 \in D2 x ->
f2 x x2 \in imset2 f2 (mem D) (fun x1 => mem (D2 x1)).