Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  a : A
  l : list A
  b : B
  l' : list B
  f : Forall2 P (a :: l) (b :: l')
  IHl : ∀ l' : list B, Forall2 P l l' → All2 P l l'
  ============================
   (P a b)


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