Goal:
  
  A : Type
  B : Type
  f : A → B
  a : A
  l : list A
  H : a :: l ≠ []
  e : map f (a :: l) = []
  ============================
   (a :: l = [])


DEBUG:Going to execute:
contradiction
Level 0: Backtrace:

Proof is not complete.
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Proof is not complete.
Level 1: Backtrace:

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

Proof is not complete.