Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  P : C → A → Prop
  fn : ∀ x : A, (∀ y : C, Q y → P y x) → B
  x : A
  xs : list A
  h : ∀ y : C, Q y → ∥ All (P y) (x :: xs) ∥
  H : (λ (l : list A) (Hl : ∀ y : C, Q y → ∥ All (P y) l ∥) (x : list B),
         ∀ (H := CoreTactics.block) (l0 : list A) 
           (Hl0 : ∀ y : C, Q y → ∥ All (P y) l0 ∥),
           {| pr1 := l; pr2 := Hl |} = {| pr1 := l0; pr2 := Hl0 |}
           → x = map_All fn l0 Hl0
             → let H0 := CoreTactics.block in #|map_All fn l0 Hl0| = #|l0|)
        xs (map_All_obligation_2 x xs h)
        (map_All fn xs (map_All_obligation_2 x xs h))
  ============================
   (∀ (l : list A) (Hl : ∀ y : C, Q y → ∥ All (P y) l ∥),
      {| pr1 := x :: xs; pr2 := h |} = {| pr1 := l; pr2 := Hl |}
      → fn x (map_All_obligation_1 x xs h)
        :: map_All fn xs (map_All_obligation_2 x xs h) = 
        map_All fn l Hl
        → let H := CoreTactics.block in #|map_All fn l Hl| = #|l|)


DEBUG:Going to execute:
let H := fresh x in
           intro
           H
Level 3: Backtrace:

The first hypothesis in the goal is not an equality.
Level 3: Backtrace:

The first hypothesis in the goal is not an equality.
Level 3: Backtrace:

The first hypothesis in the goal is not an equality.
Level 3: Backtrace:

The first hypothesis in the goal is not an equality.