Goal:
  
  A : Type
  P : A → A → Type
  Q : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  X0 : ∀ x y : A, P x y → Q x y
  X1 : OnOne2 Q tl tl'
  ============================
   (OnOne2 Q tl tl')


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