function switch_tabs(tab) {
  var tabs = document.getElementById('tabs').getElementsByTagName('li').length;
  //switch off all tabs and content
  for (i=0; i < tabs; i++) {
    document.getElementById('tab'+(i+1)).className = '';
    document.getElementById('tab'+(i+1)+'_content').style.display = 'none';
  }
  //switch on the selected tab and content
  document.getElementById(tab).className = 'selected';
  document.getElementById(tab+'_content').style.display = 'block';
}