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
  ============================
   (forall x : A, nth_error l n = Some x -> nth_error l n = nth_error l n)


DEBUG:Going to execute:
<coq-core.plugins.ltac::revert@0> $1