+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.
+