function oeffne(welches) {
   if(document.getElementById(welches).style.display == 'none') {
     document.getElementById(welches).style.display = '';
     document.getElementById('img'+welches).src = 'images/minus.gif';
   }else{
     document.getElementById(welches).style.display = 'none';
     document.getElementById('img'+welches).src = 'images/plus.gif';
   }
 };