+theorem at_div_comm: ∀f2,g2,f1,g1.
+ H_at_div f2 g2 f1 g1 → H_at_div g2 f2 g1 f1.
+#f2 #g2 #f1 #g1 #IH #jg #jf #j #Hg #Hf
+elim (IH … Hf Hg) -IH -j /2 width=3 by ex2_intro/
+qed-.
+
+theorem at_div_pp: ∀f2,g2,f1,g1.
+ H_at_div f2 g2 f1 g1 → H_at_div (↑f2) (↑g2) (↑f1) (↑g1).
+#f2 #g2 #f1 #g1 #IH #jf #jg #j #Hf #Hg
+elim (at_inv_xpx … Hf) -Hf [1,2: * |*: // ]
+[ #H1 #H2 destruct -IH
+ lapply (at_inv_xpp … Hg ???) -Hg [4: |*: // ] #H destruct
+ /3 width=3 by at_refl, ex2_intro/
+| #xf #i #Hf2 #H1 #H2 destruct
+ lapply (at_inv_xpn … Hg ????) -Hg [5: * |*: // ] #xg #Hg2 #H destruct
+ elim (IH … Hf2 Hg2) -IH -i /3 width=9 by at_push, ex2_intro/
+]
+qed-.
+
+theorem at_div_nn: ∀f2,g2,f1,g1.
+ H_at_div f2 g2 f1 g1 → H_at_div (⫯f2) (⫯g2) (f1) (g1).
+#f2 #g2 #f1 #g1 #IH #jf #jg #j #Hf #Hg
+elim (at_inv_xnx … Hf) -Hf [ |*: // ] #i #Hf2 #H destruct
+lapply (at_inv_xnn … Hg ????) -Hg [5: |*: // ] #Hg2
+elim (IH … Hf2 Hg2) -IH -i /2 width=3 by ex2_intro/
+qed-.
+
+theorem at_div_np: ∀f2,g2,f1,g1.
+ H_at_div f2 g2 f1 g1 → H_at_div (⫯f2) (↑g2) (f1) (⫯g1).
+#f2 #g2 #f1 #g1 #IH #jf #jg #j #Hf #Hg
+elim (at_inv_xnx … Hf) -Hf [ |*: // ] #i #Hf2 #H destruct
+lapply (at_inv_xpn … Hg ????) -Hg [5: * |*: // ] #xg #Hg2 #H destruct
+elim (IH … Hf2 Hg2) -IH -i /3 width=7 by at_next, ex2_intro/
+qed-.
+
+theorem at_div_pn: ∀f2,g2,f1,g1.
+ H_at_div f2 g2 f1 g1 → H_at_div (↑f2) (⫯g2) (⫯f1) (g1).
+/4 width=6 by at_div_np, at_div_comm/ qed-.
+
+(* Properties on tls ********************************************************)
+
+lemma at_pxx_tls: ∀n,f. @⦃0, f⦄ ≡ n → @⦃0, ⫱*[n]f⦄ ≡ 0.
+#n elim n -n //
+#n #IH #f #Hf
+cases (at_inv_pxn … Hf) -Hf [ |*: // ] #g #Hg #H0 destruct
+<tls_xn /2 width=1 by/
+qed.
+
+lemma at_tls: ∀i2,f. ↑⫱*[⫯i2]f ≗ ⫱*[i2]f → ∃i1. @⦃i1, f⦄ ≡ i2.
+#i2 elim i2 -i2
+[ /4 width=4 by at_eq_repl_back, at_refl, ex_intro/
+| #i2 #IH #f <tls_xn <tls_xn in ⊢ (??%→?); #H
+ elim (IH … H) -IH -H #i1 #Hf
+ elim (pn_split f) * #g #Hg destruct /3 width=8 by at_push, at_next, ex_intro/
+]
+qed-.
+
+(* Inversion lemmas with tls ************************************************)
+
+lemma at_inv_nxx: ∀n,g,i1,j2. @⦃⫯i1, g⦄ ≡ j2 → @⦃0, g⦄ ≡ n →
+ ∃∃i2. @⦃i1, ⫱*[⫯n]g⦄ ≡ i2 & ⫯(n+i2) = j2.
+#n elim n -n
+[ #g #i1 #j2 #Hg #H
+ elim (at_inv_pxp … H) -H [ |*: // ] #f #H0
+ elim (at_inv_npx … Hg … H0) -Hg [ |*: // ] #x2 #Hf #H2 destruct
+ /2 width=3 by ex2_intro/
+| #n #IH #g #i1 #j2 #Hg #H
+ elim (at_inv_pxn … H) -H [ |*: // ] #f #Hf2 #H0
+ elim (at_inv_xnx … Hg … H0) -Hg #x2 #Hf1 #H2 destruct
+ elim (IH … Hf1 Hf2) -IH -Hf1 -Hf2 #i2 #Hf #H2 destruct
+ /2 width=3 by ex2_intro/
+]
+qed-.
+
+lemma at_inv_tls: ∀i2,i1,f. @⦃i1, f⦄ ≡ i2 → ↑⫱*[⫯i2]f ≗ ⫱*[i2]f.
+#i2 elim i2 -i2
+[ #i1 #f #Hf elim (at_inv_xxp … Hf) -Hf // #g #H1 #H destruct
+ /2 width=1 by eq_refl/
+| #i2 #IH #i1 #f #Hf
+ elim (at_inv_xxn … Hf) -Hf [1,3: * |*: // ]
+ [ #g #j1 #Hg #H1 #H2 | #g #Hg #Ho ] destruct
+ <tls_xn /2 width=2 by/
+]