Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Prop
  p : A → B → C → bool
  Hp : ∀ (x : A) (y : B) (z : C), reflect (P x y z) (p x y z)
  x : A
  y : B
  z : C
  l : list A
  l' : list B
  l'' : list C
  H : P x y z
  H0 : Forall3 P l l' l''
  IHForall3 : is_true (forallb3 p l l' l'')
  ============================
   (p x y z && true = true)


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.