Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  R : C → A → Prop
  f : ∀ x : A, (∀ y : C, Q y → R y x) → B
  x : A
  xs : list A
  h : ∀ y : C, Q y → ∥ All (R y) (x :: xs) ∥
  X : ∀ (P : C → A → Prop) (Q' : B → Type),
        (∀ y : C, Q y → All (P y) xs)
        → (∀ (x : A) (y : C) (rx : ∀ y0 : C, Q y0 → R y0 x),
             P y x → Q' (f x rx))
          → ∀ y : C,
              Q y → All Q' (map_All f xs (map_All_obligation_2 x xs h))
  P : C → A → Prop
  Q' : B → Type
  ha : ∀ y : C, Q y → All (P y) (x :: xs)
  hf : ∀ (x : A) (y : C) (rx : ∀ y0 : C, Q y0 → R y0 x), P y x → Q' (f x rx)
  y : C
  hy : Q y
  p : P y x
  X0 : All (P y) xs
  y0 : C
  X1 : All (P y0) (x :: xs)
  X2 := {| pr1 := x :: xs; pr2 := X1 |}
     : sigma (λ index : list A, All (P y0) index)
  X3 := x :: xs : list A
  X4 := X1 : (λ index : list A, All (P y0) index) X3
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All (P y0) xs)


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.