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'
  X5 : 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:
fun flags =>
             match reverse goal with
             | |- ?X1 =>
                   <Coq.Init.Tauto.is_unit_or_eq> flags X1; constructor 1
             | H:?X1 |- _ => <Coq.Init.Tauto.is_empty> flags X1; elim H
             | _ => Ltac.assumption
             end