Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Prop
  p : A → B → C → bool
  b : B
  l' : list B
  Hp : ∀ (x : A) (y : B) (z : C), reflect (P x y z) (p x y z)
  H : is_true false
  ============================
   (Forall3 P [] (b :: l') [])


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.