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


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>