(* Basic_2A1: includes: drop_mono *)
lemma drops_mono: ∀b1,f,L,L1. ⇩*[b1,f] L ≘ L1 →
∀b2,L2. ⇩*[b2,f] L ≘ L2 → L1 = L2.
-#b1 #f #L #L1 lapply (after_isid_dx ð\9d\90\88ð\9d\90\9d … f)
+#b1 #f #L #L1 lapply (after_isid_dx ð\9d\90¢ … f)
/3 width=8 by drops_conf, drops_fwd_isid/
qed-.