function partOver() { 
  document.particulier.src ="./images/buttons/partover.gif"; }
function partOut() { 
  document.particulier.src ="./images/buttons/part.gif"; }

function zakOver() { 
  document.zakelijk.src ="./images/buttons/zakover.gif"; }
function zakOut() {
  document.zakelijk.src ="./images/buttons/zak.gif"; }

function clearOver() { 
  document.particulier.src ="./images/buttons/part.gif";
  document.zakelijk.src ="./images/buttons/zak.gif";
  }