Goal:
  
  A : Type
  B : Type
  C : Type
  p : A → B → C → bool
  c : C
  l'' : list C
  h : is_true (forallb3 p [] [] (c :: l''))
  ============================
   (Forall3 (λ (x : A) (y : B) (z : C), p x y z) [] [] (c :: l''))


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

No primitive equality found.
Level 0: Backtrace:

No primitive equality found.