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 ∥)
  l0 : list A
  n : nat
  a : A
  l : list A
  Heq_l : a :: l = l0
  allq : ∥ All Q (a :: l) ∥
  X : ∥ P n a ∥
  ============================
   (∥ All Q l ∥)


DEBUG:Going to execute:
subst Y
Level 1: Backtrace:

Ltac variable X is bound to a :: l of type constr which cannot be coerced to
a variable.