Goal:
  
  A : Type
  P : A → A → Type
  Q : A → A → Type
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  HPQ : ∀ x y : A, P x y → ∑ z : A, Q x z × Q y z
  x : A
  q : Q hd x
  q0 : Q hd' x
  ============================
   (OnOne2 Q (hd :: tl) (x :: tl) × OnOne2 Q (hd' :: tl) (x :: tl))


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