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
  X : All_fold P (a :: Γ ++ Δ)
  X0 := {| pr1 := a :: Γ ++ Δ; pr2 := X |}
     : sigma (λ index : list A, All_fold P index)
  X1 := a :: Γ ++ Δ : list A
  X2 := X : (λ index : list A, All_fold P index) X1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All_fold (λ Γ : list A, P (Γ ++ Δ)) (a :: Γ))


DEBUG:Going to execute:
match goal with
           | id'1:=?t:_
             |- _ =>
                 generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear
                  id' id; try unfold signature in id'2; 
                  hnf in id'2; simpl in id'2; rename id'2 into id;
                  cont id id id'1 t
           end
Level 1: Backtrace:

No head constant to reduce.