Goal:
  
  A : Type
  P : A → A → Type
  Q : A → Type
  R : A → Type
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  X0 : Q hd
  X1 : All Q tl
  X2 : R hd
  X3 : All R tl'
  X4 : All R tl' → OnOne2 (λ x y : A, P x y × Q x × R y) tl tl'
  ============================
   (OnOne2 (λ x y : A, P x y × Q x × R y) tl tl')


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