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
  map_All : ∀ l : list A, (∀ y : C, Q y → ∥ All (P y) l ∥) → list B
  x : A
  ============================
   (∀ xs : list A,
      (∀ y : C, Q y → ∥ All (P y) (x :: xs) ∥)
      → ∀ y : C, Q y → ∥ All (P y) xs ∥)


DEBUG:Going to execute:
let H := fresh x in
           intro
           H
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.