04.Answer

def tally[O2 >: O](using m: Monoid[O2]): Pull[O2, R] =
  def go(total: O2, p: Pull[O, R]): Pull[O2, R] =
    p.uncons.flatMap:
      case Left(r) => Result(r)
      case Right((hd, tl)) =>
        val newTotal = m.combine(total, hd)
        Output(newTotal) >> go(newTotal, tl)
  Output(m.empty) >> go(m.empty, this)