+lemma drops_inv_isuni: ∀L1,L2,f. ⬇*[Ⓣ, f] L1 ≡ L2 → 𝐔⦃f⦄ →
+ (𝐈⦃f⦄ ∧ L2 = L1) ∨
+ ∃∃I,K,V,g. ⬇*[Ⓣ, g] K ≡ L2 & L1 = K.ⓑ{I}V & f = ⫯g.
+#L1 #L2 #f * -L1 -L2 -f
+[ /4 width=1 by or_introl, conj/
+| /4 width=7 by ex3_4_intro, or_intror/
+| /7 width=6 by drops_fwd_isid, lifts_fwd_isid, isuni_inv_push, isid_push, or_introl, conj, eq_f3, sym_eq/
+]
+qed-.
+