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:
<coq-core.plugins.ltac::discriminate@0>
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Proof is not complete.
Level 0: Backtrace:

Proof is not complete.
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Proof is not complete.
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Proof is not complete.
Level 0: Backtrace:

Proof is not complete.