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
  h : ∀ y : C, Q y → ∥ All (P y) (x :: xs) ∥
  y : C
  X : Q y
  ============================
   (P y x)


DEBUG:Going to execute:
match H with
           | H' => fail 2
           | _ => unify X Y; clear H' || clear H
           end
Level 4: Backtrace:

A is used in hypothesis P.
Level 4: Backtrace:

A is used in hypothesis P.
Level 4: Backtrace:

A is used in hypothesis P.
Level 4: Backtrace:

A is used in hypothesis P.
Level 4: Backtrace:

A is used in hypothesis P.
Level 4: Backtrace:

A is used in hypothesis P.
Level 4: Backtrace:

A is used in hypothesis P.
Level 3: Backtrace:

A is used in hypothesis P.