Goal:
  
  T : Type -> Type
  M : Monad T
  A : Type
  P : nat -> A -> Type
  k : nat
  a : A
  l : list A
  f : forall (n : nat) (x : A),
      nth_error (a :: l) n = Some x -> T (∥ P (k + n) x ∥)
  X : ∥ P (k + 0) a ∥
  n : nat
  x : A
  hnth : nth_error l n = Some x
  px : ∥ P (k + S n) x ∥
  H0 : nth_error l n = Some x
  ============================
   (∥ P (S (k + n)) x ∥)


DEBUG:Going to execute:
subst X