Goal:
  
  ============================
   (forall T : Type -> Type,
    Monad T ->
    forall (A : Type) (P : nat -> A -> Type) (Q : A -> Type),
    (forall (n : nat) (x : A), ∥ Q x ∥ -> T (∥ P n x ∥)) ->
    forall (n : nat) (a : A) (l : list A),
    ∥ All Q (a :: l) ∥ -> ∥ P n a ∥ -> ∥ All Q l ∥)


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.