code: public boolean barIsEmptyForAllVoices(Bar bar) {
          for (Object m_voice : m_voices) {
              Voice v = (Voice) m_voice;
              if (!v.barIsEmpty(bar))
                  return false;
          }
  		return true;
  	}
docs: Return true if the bar is empty or contains only barline and spacer(s).
  False if barline contain other kind of music element