Goal:
  
  A : Type
  f : nat → A
  m : nat
  t : A
  IHm : ∀ n : nat, nth_error (unfold m f) n = Some t → t = f n
  H : m < m + 1
  Hn : Some (f m) = Some t
  ============================
   (t = 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.