function show_tab( obj_id, nav_id ){
	if ( !document.getElementById ){
		return ;
	}
	var i , obj , parent_node ;
	if ( document.getElementById( obj_id ) ) {
		obj = document.getElementById( obj_id ) ;
		obj_list = obj.parentNode.getElementsByTagName( obj.tagName ) ;
		for ( i = 0 ; i < obj_list.length ; i++ ){
			obj_list.item(i).style.display = "none" ;
		}
		obj.style.display = "block" ;
		for ( i = 1; i < 6; i++ ) {
			document.getElementById('nav'+i).className = obj_id;
		}
		document.getElementById(nav_id).className = obj_id+'_hover';
	}
}