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
  ============================
   (nth_error [f m] (m - m) = Some (f m))


DEBUG:Going to execute:
match goal with
           | H:¬ _ |- _ => solve [ case H; trivial ]
           end
Level 1: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.