Goal:
    A : Type  d : A
  ============================
   (∀ (i : nat) (v : A),
      nth i [] d = v
      → (nth_error [] i = Some v) + (nth_error [] i = None ∧ v = d))


DEBUG:Going to execute:
case  =>/= //
Level 0: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 1: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.