Goal:
  
  A : Type
  f : nat → A
  m : nat
  IHm : ∀ n : nat, n < m ↔ nth_error (unfold m f) n = Some (f n)
  Hn : m < S m
  ============================
   (Some (f m) = Some (f m))


DEBUG:Going to execute:
done
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.
DEBUG LTAC Evaluated term: Nat.sub_diag
DEBUG LTAC Evaluated term: Nat.sub_diag
DEBUG LTAC Evaluated term: Prop
DEBUG LTAC Evaluated term: Prop
DEBUG LTAC Evaluated term: Prop
DEBUG LTAC Evaluated term: Prop