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 ∥)
  ============================
   (Some a = Some a)


DEBUG:Going to execute:
match goal with
           | H:?X = ?Y |- _ => subst X || subst Y
           end