Goal:
  
  ============================
   (∀ (A B 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.