Goal:
  
  A : Type
  B : Type
  f : nat → A
  a : B
  l : list B
  b : B
  l' : list B
  IHl : ∀ (f : nat → A) (l' : list B),
          #|l| = #|l'|
          → mapi (λ (i : nat) (_ : B), f i) l =
            mapi (λ (i : nat) (_ : B), f i) l'
  H : #|a :: l| = #|b :: l'|
  ============================
   (mapi (λ (i : nat) (_ : B), f i) (a :: l) =
    mapi (λ (i : nat) (_ : B), f i) (b :: l'))


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>
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Proof is not complete.
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Proof is not complete.
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Proof is not complete.