Goal:
  
  A : Type
  P : A → A → Type
  Q : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  H : OnOne2 P tl tl'
  HPQ : ∀ x y : A, P x y → ∑ z : A, Q x z × Q y z
  r : list A
  o : OnOne2 Q tl r
  o0 : OnOne2 Q tl' r
  ============================
   (OnOne2 Q tl r)


DEBUG:Going to execute:
(solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"