Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Prop
  a : A
  l : list A
  f : Forall3 P (a :: l) [] []
  IHl : ∀ (l' : list B) (l'' : list C), Forall3 P l l' l'' → All3 P l l' l''
  ============================
   False


DEBUG:Going to execute:
inversion_clear H
Level 1: evaluation returns
inversion_clear H
where
H := f
of type
intropattern of type tacvalue