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


DEBUG:Going to execute:
intro || (let H := fresh in
                     intro
                     H)
DEBUG LTAC Evaluated term: (∥ Q a ∥)
DEBUG LTAC Evaluated term: Prop