public RhythmGroup addOverlay(RhythmOverlay overlay){
  mOverlays.add(overlay);
  if (mCurrentOverlayIndex == NO_OVERLAY) {
    selectOverlay(0);
  }
  return this;
}

public RhythmGroup addOverlay ( RhythmOverlay overlay ) { mOverlays . add ( overlay ) ; if ( mCurrentOverlayIndex == NO_OVERLAY ) { selectOverlay ( 0 ) ; } return this ; }
add rhythm overlay to this group
