Goal:
  
  A : Type
  P : list A → A → Type
  Q : list A → list A → A → A → Type
  Δ : list A
  hlen : #|[]| = #|Δ|
  hb : All_fold P Δ
  H : ∀ (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 [] Δ)


DEBUG:Going to execute:
tac H