Goal:
  
  A : Type
  P : A → A → Prop
  r : A
  r' : A
  H : Forall2 P [r] [r']
  H3 : P r r'
  H5 : Forall2 P [] []
  ============================
   (Forall2 P [] [])


DEBUG:Going to execute:
match goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end