lapply (frees_inv_lref_skip … H ?) -L //
| elim (lt_or_ge j (|L|)) #Hj
[ elim (drop_O1_lt (Ⓕ) L j) // -Hj #I #K #W #HLK destruct
elim (IH K W … 0 (i-j-1)) -IH [1,3: /3 width=5 by frees_lref_be, drop_fwd_rfw, or_introl/ ] #HnW
@or_intror #H elim (frees_inv_lref_lt … H) // #Z #Y #X #_ #HLY -l
lapply (drop_mono … HLY … HLK) -L #H destruct /2 width=1 by/
lapply (frees_inv_lref_skip … H ?) -L //
| elim (lt_or_ge j (|L|)) #Hj
[ elim (drop_O1_lt (Ⓕ) L j) // -Hj #I #K #W #HLK destruct
elim (IH K W … 0 (i-j-1)) -IH [1,3: /3 width=5 by frees_lref_be, drop_fwd_rfw, or_introl/ ] #HnW
@or_intror #H elim (frees_inv_lref_lt … H) // #Z #Y #X #_ #HLY -l
lapply (drop_mono … HLY … HLK) -L #H destruct /2 width=1 by/