elim (ldrop_inv_O1_pair1 … H) -H * #Hi #HLK1 [ -IHL12 | -HL12 ]
[ #_ destruct -I2 >ypred_succ
/2 width=4 by ldrop_pair, ex2_2_intro/
| lapply (ylt_inv_O1 i ?) /2 width=1 by ylt_inj/
#H <H -H #H lapply (ylt_inv_succ … H) -H
#Hie elim (IHL12 … HLK1) -IHL12 -HLK1 // -Hie
elim (ldrop_inv_O1_pair1 … H) -H * #Hi #HLK1 [ -IHL12 | -HL12 ]
[ #_ destruct -I2 >ypred_succ
/2 width=4 by ldrop_pair, ex2_2_intro/
| lapply (ylt_inv_O1 i ?) /2 width=1 by ylt_inj/
#H <H -H #H lapply (ylt_inv_succ … H) -H
#Hie elim (IHL12 … HLK1) -IHL12 -HLK1 // -Hie