d2 ≤ d1 → ↑[d2,h2]↑[d1,h1]T = ↑[h2+d1,h1]↑[d2,h2]T.
/3 width=1 by flifts_comp, basic_swap/ qed-.
(*
-lemma flift_join: â\88\80e1,e2,T. â¬\86[e1,e2] ↑[0,e1] T ≡ ↑[0,e1 + e2] T.
+lemma flift_join: â\88\80e1,e2,T. â\87§[e1,e2] ↑[0,e1] T ≡ ↑[0,e1 + e2] T.
#e1 #e2 #T
lapply (flift_lift T 0 (e1+e2)) #H
elim (lift_split … H e1 e1) -H // #U #H