Goal:
  
  A : Type
  P : A → A → Type
  a : A
  l : list A
  a0 : A
  l' : list A
  r : A
  r' : A
  IHl : ∀ l' : list A, All2 P (l ++ [r]) (l' ++ [r']) → All2 P l l' × P r r'
  X : All2 P (a :: l ++ [r]) ((a0 :: l') ++ [r'])
  X0 : P a a0
  a1 : All2 P l l'
  b : P r r'
  ============================
   (P a a0)


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