Goal:
  
  A : Type
  P : A → A → Prop
  a : A
  l : list A
  a0 : A
  l' : list A
  r : A
  r' : A
  H : Forall2 P (a :: l ++ [r]) (a0 :: l' ++ [r'])
  H3 : P a a0
  H5 : Forall2 P (l ++ [r]) (l' ++ [r'])
  H0 : Forall2 P l l'
  H1 : P r r'
  ============================
   (Forall2 P (a :: l) (a0 :: l'))


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