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


DEBUG:Going to execute:
let H := fresh x in
           intro
           H
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.
Level 2: Backtrace:

The first hypothesis in the goal is not an equality.