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


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