@⦃i, des⦄ ≡ i0 & des ▭ i ≡ des0.
#L1 #L #des #H elim H -L1 -L -des
[ /2 width=7 by drops_nil, minuss_nil, at_nil, ex4_3_intro/
-| #L1 #L0 #L #des #d #e #_ #HL0 #IHL0 #L2 #i #HL2
- elim (lt_or_ge i d) #Hid
+| #L1 #L0 #L #des #l #m #_ #HL0 #IHL0 #L2 #i #HL2
+ elim (lt_or_ge i l) #Hil
[ elim (drop_trans_le … HL0 … HL2) -L /2 width=2 by lt_to_le/
#L #HL0 #HL2 elim (IHL0 … HL0) -L0 /3 width=7 by drops_cons, minuss_lt, at_lt, ex4_3_intro/
| lapply (drop_trans_ge … HL0 … HL2 ?) -L // #HL02