#L1 #L2 * #L #HL1 #HL2 #V1 #V2 *
<(ltpss_sn_fwd_length … HL2) #V #HV1 #HV2 #I
lapply (ltpss_sn_tpss_trans_eq … HV2 … HL2) -HV2 #V2
-@(ex2_1_intro … (L.ⓑ{I}V)) /2 width=1/ (**) (* explicit constructor *)
+@(ex2_intro … (L.ⓑ{I}V)) /2 width=1/ (**) (* explicit constructor *)
qed.