def fast_forward(dt)
      adjusted_dt = dt * @relative_rate
      @now += adjusted_dt
      @children.each { |sub_clock| sub_clock.fast_forward(adjusted_dt) }
    end
def fast_forward(dt)
      adjusted_dt = dt * @relative_rate
      @now += adjusted_dt
      @children.each { |sub_clock| sub_clock.fast_forward(adjusted_dt) }
    end
fast-forward this clock and all children clocks by the given time delta