Goal:
  
  A : Type
  P : A → A → Type
  l' : list A
  f : A → A
  hp : ∀ x y : A, P (f x) y → ∑ y' : A, y = f y'
  o : OnOne2 P (map f []) l'
  ============================
   (let H := CoreTactics.block in
    ∑ l'' : list A, OnOne2 (λ x y : A, P (f x) (f y)) [] l'')


DEBUG:Going to execute:
fun id =>
             CoreTactics.on_last_hyp
              ltac:(fun id' =>
                      match id' with
                      | id => idtac
                      | _ => revert id'; CoreTactics.revert_until id
                      end)