Goal:
    T : Type -> Type  M : 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:
<coq-core.plugins.ltac::intro@1> $1