Goal:
    T : Type -> Type  M : Monad T  A : Type  P : nat -> A -> Type  k : nat
  ============================
   (forall (a : A) (l : list A),
    (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:
<coq-core.plugins.ltac::intro@1> $1