-axiom lsubf_eq_repl_back1: ∀f2,L1,L2. eq_repl_back … (λf1. ⦃L1,f1⦄ ⫃𝐅+ ⦃L2,f2⦄).
+lemma lsubf_eq_repl_back1: ∀f2,L1,L2. eq_repl_back … (λf1. ⦃L1,f1⦄ ⫃𝐅+ ⦃L2,f2⦄).
+#f2 #L1 #L2 #f #H elim H -f -f2 -L1 -L2
+[ #f1 #f2 #Hf12 #g1 #Hfg1
+ /3 width=3 by lsubf_atom, eq_canc_sn/
+| #f1 #f2 #I1 #I2 #K1 #K2 #_ #IH #g #H
+ elim (eq_inv_px … H) -H [|*: // ] #g1 #Hfg1 #H destruct
+ /3 width=1 by lsubf_push/
+| #f1 #f2 #I #K1 #K2 #_ #IH #g #H
+ elim (eq_inv_nx … H) -H [|*: // ] #g1 #Hfg1 #H destruct
+ /3 width=1 by lsubf_bind/
+| #f #f0 #f1 #f2 #K1 #L2 #W #V #Hf #Hf1 #_ #IH #g #H
+ elim (eq_inv_nx … H) -H [|*: // ] #g1 #Hfg1 #H destruct
+ /3 width=5 by lsubf_beta, sor_eq_repl_back3/
+| #f #f0 #f1 #f2 #I1 #I2 #K1 #K2 #V #Hf #Hf1 #_ #IH #g #H
+ elim (eq_inv_nx … H) -H [|*: // ] #g1 #Hfg1 #H destruct
+ /3 width=5 by lsubf_unit, sor_eq_repl_back3/
+]
+qed-.