definition subset_ext_f1 (A1) (A0) (f:A1→A0): 𝒫❨A1❩ → 𝒫❨A0❩ ≝
λu1,a0. ∃∃a1. a1 ϵ u1 & f a1 = a0.
+definition subset_ext_f1_1 (A11) (A21) (A0) (f1:A11→A0) (f2:A21→A0): 𝒫❨A11❩ → 𝒫❨A21❩ → 𝒫❨A0❩ ≝
+ λu11,u21,a0.
+ ∨∨ subset_ext_f1 A11 A0 f1 u11 a0
+ | subset_ext_f1 A21 A0 f2 u21 a0.
+
definition subset_ext_p1 (A1) (Q:predicate A1): predicate (𝒫❨A1❩) ≝
λu1. ∀a1. a1 ϵ u1 → Q a1.
a1 ϵ u1 → f a1 ϵ subset_ext_f1 A1 A0 f u1.
/2 width=3 by ex2_intro/ qed.
+lemma subset_in_ext_f1_1_dx_1 (A11) (A21) (A0) (f1) (f2) (u11) (u21) (a11):
+ a11 ϵ u11 → f1 a11 ϵ subset_ext_f1_1 A11 A21 A0 f1 f2 u11 u21.
+/3 width=3 by subset_in_ext_f1_dx, or_introl/ qed.
+
+lemma subset_in_ext_f1_1_dx_2 (A11) (A21) (A0) (f1) (f2) (u11) (u21) (a21):
+ a21 ϵ u21 → f2 a21 ϵ subset_ext_f1_1 A11 A21 A0 f1 f2 u11 u21.
+/3 width=3 by subset_in_ext_f1_dx, or_intror/ qed.
+
(* Basic inversions *********************************************************)
lemma subset_in_inv_ext_p1_dx (A1) (Q) (u1) (a1):