/3 width=3 by subset_inclusion_ext_f1_bi, conj/
qed.
+lemma subset_equivalence_ext_f1_1_bi (A11) (A21) (A0) (f1) (f2) (u11) (u21) (v11) (v21):
+ u11 ⇔ v11 → u21 ⇔ v21 →
+ subset_ext_f1_1 A11 A21 A0 f1 f2 u11 u21 ⇔ subset_ext_f1_1 A11 A21 A0 f1 f2 v11 v21.
+#A11 #A21 #A0 #f1 #f2 #u11 #u21 #v11 #v21 * #Huv11 #Hvu11 * #Huv21 #Hvu21
+/3 width=5 by subset_inclusion_ext_f1_1_bi, conj/
+qed.
+
lemma subset_inclusion_ext_f1_compose (A0) (A1) (A2) (f1) (f2) (u):
subset_ext_f1 A1 A2 f2 (subset_ext_f1 A0 A1 f1 u) ⇔ subset_ext_f1 A0 A2 (f2∘f1) u.
/3 width=1 by subset_inclusion_ext_f1_compose_dx, subset_inclusion_ext_f1_compose_sn, conj/