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
  X : P n a
  ============================
   (All Q (a :: l) -> ∥ All Q l ∥)


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