- l = 0 → m ≤ |L1| →
- ∀L. ⬇[s, 0, m] L @@ L1 ≡ L @@ L2.
-#L1 #L2 #s #l #m #H elim H -L1 -L2 -l -m normalize
-[2,3,4: /4 width=1 by drop_skip_lt, drop_drop, arith_b1, lt_minus_to_plus_r, monotonic_pred/ ]
-#l #m #_ #_ #H <(le_n_O_to_eq … H) -H //
+ l = yinj 0 → m ≤ |L1| →
+ ∀L. ⬇[s, yinj 0, m] L @@ L1 ≡ L @@ L2.
+#L1 #L2 #s #l #m #H elim H -L1 -L2 -l -m //
+[ #l #m #_ #_ #H <(le_n_O_to_eq … H) -H //
+| normalize /4 width=1 by drop_drop, monotonic_pred/
+| #I #L1 #L2 #V1 #V2 #l #m #_ #_ #_ #H elim (ysucc_inv_O_dx … H)
+]