function hand_rechts_aan(){
  document.getElementById('right_overlay_hand').style.display='';
}
function hand_rechts_uit(){
  document.getElementById('right_overlay_hand').style.display='none';
}
function hand_links_aan(){
  document.getElementById('left_overlay_hand').style.display='';
}
function hand_links_uit(){
  document.getElementById('left_overlay_hand').style.display='none';
}