Goal:
  
  A : Type
  P : list A → A → Type
  a : A
  Γ : list A
  Δ : list A
  IHΓ : ∀ Δ : list A,
          All_fold P (Γ ++ Δ)
          → All_fold P Δ × All_fold (λ Γ : list A, P (Γ ++ Δ)) Γ
  X1 : list A
  X : All_fold P X1
  ============================
   (a :: Γ ++ Δ = X1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All_fold P Δ)


DEBUG:Going to execute:
cbv beta iota delta [TODO projection TODO projection] in id, id'
DEBUG LTAC Evaluated term: X1
DEBUG LTAC Evaluated term: (list A)
Level 3: type of id
 has value 
(list A)