function toggle(elm) {
  if(document.getElementById(elm).style.display == "block"){
    document.getElementById(elm).style.display = "none";
  } else {
    document.getElementById(elm).style.display = "block";
  }
}

function toggleColor(elm) {
  if(document.getElementById(elm).className == "lichtblokje"){
    document.getElementById(elm).className = "witblokje";
  } else {
    document.getElementById(elm).className = "lichtblokje";
  }
}


function openWindow(url, name, width, height) {
  left = (screen.width  - width)  / 2;
  top  = (screen.height - height) / 2;
  return window.open(url, name, 'width='+width+',height='+height+',scrollbars=yes,toolbar=no,location=no,top='+top+',left='+left);
}