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


DEBUG:Going to execute:
<coq-core.plugins.ltac::intro@1> $1