elim (append_inj_dx … H ?) -H // -HX #_ #H destruct -X
lapply (ltpss_sn_fwd_length … HL2) >append_length >append_length #H
lapply (injective_plus_r … H) -H #H
- @(ex3_1_intro … (⋆.ⓑ{I}V@@Y)) <append_assoc // -HT12
+ @(ex3_intro … (⋆.ⓑ{I}V@@Y)) <append_assoc // -HT12
<append_assoc [ /3 width=1/ ] -HV1 -HY
>append_length <associative_plus
@(ltpss_sn_dx_trans_eq … HL2) -HL2 >H -H >commutative_plus /3 width=1/