-lemma pippo (h) (a) (G) (L0):
- ∀T0. ⦃G,L0⦄ ⊢ T0 ![h,a] →
- ∀n,T1. ⦃G,L0⦄ ⊢ T0 ➡[n,h] T1 → ∀T2. ⦃G,L0⦄ ⊢ T0 ⬌η[h] T2 →
- ∀L1. ⦃G,L0⦄ ⊢ ➡[h] L1 →
- ∃∃T. ⦃G,L1⦄ ⊢ T1 ⬌η[h] T & ⦃G,L0⦄ ⊢ T2 ➡[n,h] T.
-#h #a #G #L0 * *
-[ #s #_ #n #X1 #HX1 #X2 #HX2 #L1 #HL01
+definition dropable_bi: predicate … ≝
+ λR. ∀L1,L2. L1 ⪤[R] L2 → ∀b,f. 𝐔⦃f⦄ →
+ ∀K1. ⇩*[b,f] L1 ≘ K1 → ∀K2. ⇩*[b,f] L2 ≘ K2 → K1 ⪤[R] K2.
+
+definition IH (h) (a): relation3 genv lenv term ≝
+ λG,L0,T0. ⦃G,L0⦄ ⊢ T0 ![h,a] →
+ ∀n,T1. ⦃G,L0⦄ ⊢ T0 ➡[n,h] T1 → ∀T2. ⦃G,L0⦄ ⊢ T0 ⬌η[h] T2 →
+ ∀L1. ⦃G,L0⦄ ⊢ ➡[h] L1 →
+ ∃∃T. ⦃G,L1⦄ ⊢ T1 ⬌η[h] T & ⦃G,L0⦄ ⊢ T2 ➡[n,h] T.
+
+lemma pippo_aux (h) (a) (G0) (L0) (T0):
+ (∀G,L,T. ⦃G0,L0,T0⦄ >[h] ⦃G,L,T⦄ → IH h a G L T) →
+ IH h a G0 L0 T0.
+#h #a #G0 #L0 * *
+[ #s #_ #_ #n #X1 #HX1 #X2 #HX2 #L1 #HL01