Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Q : list A → list A → A → A → Type
  h : All2_fold Q [] []
  h0 := {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := h |}
     : sigma
         (λ index : sigma (λ _ : list A, list A),
            All2_fold Q (pr1 index) (pr2 index))
  h1 := {| pr1 := []; pr2 := [] |} : sigma (λ _ : list A, list A)
  h2 := h
     : (λ index : sigma (λ _ : list A, list A),
          All2_fold Q (pr1 index) (pr2 index)) h1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2_fold (λ (Γ Δ : list A) (x y : A), P Γ Δ x y × Q Γ Δ x y) [] [])


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.