Goal:
  
  A : Type
  P : list A → A → Type
  Q : list A → list A → A → A → Type
  hlen : #|[]| = #|[]|
  ============================
   (let H := CoreTactics.block in
    (∀ (Δ Δ' : list A) (x y : A),
       All_fold P Δ
       → All_fold P Δ' → All2_fold Q Δ Δ' → P Δ x → P Δ' y → Q Δ Δ' x y)
    → let H0 := CoreTactics.block in All2_fold Q [] [])


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
((∀ (Δ Δ' : list A) (x y : A),
    All_fold P Δ
    → All_fold P Δ' → All2_fold Q Δ Δ' → P Δ x → P Δ' y → Q Δ Δ' x y)
 → let H := CoreTactics.block in All2_fold Q [] [])