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
  ============================
   (nth_error l n = nth_error l n)


DEBUG:Going to execute:
try Tactics.discriminates
Level 6: Backtrace:

Tactic failure (level 2).
Level 6: Backtrace:

Tactic failure (level 1).
Level 5: Backtrace:

Tactic failure (level 1).
Level 4: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 3: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 3: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 3: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 3: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.
Level 1: Backtrace:

No matching clauses for match.
Level 1: evaluation
raised the exception
Backtrace:

No matching clauses for match.

Level 2: Backtrace:

No matching clauses for match.
Level 1: Backtrace:

No matching clauses for match.
Level 1: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.
Level 1: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.