Goal:
    A : Type  B : Type  C : Type  Q : C → Type
  ============================
   (∀ P : C → A → Prop,
      (∀ x : A, (∀ y : C, Q y → P y x) → B)
      → (∀ 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.