function switchToTab( tab ) {
  var tabIdString;
  var tabContentIdString;
  var numOfTabs = 5;
  for( var i = 1 ; i <= numOfTabs ; i++ ) {
    tabIdString = 'tab_' + i;
    tabEndIdString = 'tab_' + i + '_end';
    tabContentIdString = 'tab_' + i + '_content';
    if( i == tab ) {	/* the selected tab & the selected content */
      document.getElementById(tabIdString).className = 'tab_selected';
      document.getElementById(tabEndIdString).className = 'tab_selected_end';
      document.getElementById(tabContentIdString).style.display = 'block';
    } else {		/* non-selected tab & content */
      document.getElementById(tabIdString).className = 'tab';
      document.getElementById(tabEndIdString).className = 'tab_end';
      document.getElementById(tabContentIdString).style.display = 'none';
    }
  }
  return null;
}