function change(which) {
   document.getElementById('div1').style.display = 'none';
   document.getElementById('div2').style.display = 'none';
   document.getElementById('div3').style.display = 'none';
   document.getElementById('div4').style.display = 'none';
   document.getElementById(which).style.display = 'block';
   }

function showall() {
   document.getElementById('div1').style.display = 'block';
   document.getElementById('div2').style.display = 'block';
   document.getElementById('div3').style.display = 'block';
   document.getElementById('div4').style.display = 'block';
   }