+ lapply (lift_inv_flat2 … H) -H * #W2 #U2 #HW2 #HU2 #H destruct
+ elim (IHW … HW2 ?) // -IHW -HW2 #W0 #HW2 #HW1
+ elim (IHU … HU2 ?) // /3 width=5/
+]
+qed.
+
+(* Note: apparently this was missing in Basic_1 *)
+theorem lift_div_be: ∀d1,e1,T1,T. ⇧[d1, e1] T1 ≡ T →
+ ∀e,e2,T2. ⇧[d1 + e, e2] T2 ≡ T →
+ e ≤ e1 → e1 ≤ e + e2 →
+ ∃∃T0. ⇧[d1, e] T0 ≡ T2 & ⇧[d1, e + e2 - e1] T0 ≡ T1.
+#d1 #e1 #T1 #T #H elim H -d1 -e1 -T1 -T
+[ #k #d1 #e1 #e #e2 #T2 #H >(lift_inv_sort2 … H) -H /2 width=3/
+| #i #d1 #e1 #Hid1 #e #e2 #T2 #H #He1 #He1e2
+ >(lift_inv_lref2_lt … H) -H [ /3 width=3/ | /2 width=3/ ]
+| #i #d1 #e1 #Hid1 #e #e2 #T2 #H #He1 #He1e2
+ elim (lt_or_ge (i+e1) (d1+e+e2)) #Hie1d1e2
+ [ elim (lift_inv_lref2_be … H ? ?) -H // /2 width=1/
+ | >(lift_inv_lref2_ge … H ?) -H //
+ lapply (le_plus_to_minus … Hie1d1e2) #Hd1e21i
+ elim (le_inv_plus_l … Hie1d1e2) -Hie1d1e2 #Hd1e12 #He2ie1
+ @ex2_1_intro [2: /2 width=1/ | skip ] -Hd1e12
+ @lift_lref_ge_minus_eq [ >plus_minus_commutative // | /2 width=1/ ]
+ ]
+| #p #d1 #e1 #e #e2 #T2 #H >(lift_inv_gref2 … H) -H /2 width=3/
+| #I #V1 #V #T1 #T #d1 #e1 #_ #_ #IHV1 #IHT1 #e #e2 #X #H #He1 #He1e2
+ elim (lift_inv_bind2 … H) -H #V2 #T2 #HV2 #HT2 #H destruct
+ elim (IHV1 … HV2 ? ?) -V // >plus_plus_comm_23 in HT2; #HT2
+ elim (IHT1 … HT2 ? ?) -T // -He1 -He1e2 /3 width=5/
+| #I #V1 #V #T1 #T #d1 #e1 #_ #_ #IHV1 #IHT1 #e #e2 #X #H #He1 #He1e2
+ elim (lift_inv_flat2 … H) -H #V2 #T2 #HV2 #HT2 #H destruct
+ elim (IHV1 … HV2 ? ?) -V //
+ elim (IHT1 … HT2 ? ?) -T // -He1 -He1e2 /3 width=5/