function flipStayPlay(which) {
  var show = which;
  var hide;
  var button1;
  var button2;
  switch (which) {
    case 'play':
      hide="stay";
    break;
    case 'stay':
      hide="play";
    break;
    default: return;
  }
  document.getElementById(hide).style.display="none";
  document.getElementById(show).style.display="block";
  document.getElementById(show + "_button").className = show + "_button_on";
  document.getElementById(hide + "_button").className = hide + "_button_off";
}