function popup(url,windowname,width,height) {
 var features="location=no, menubar=no, status=no, scrollbars=yes, resizable=yes, toolbar=no";
 if (width) {
 if (window.screen.width > width)
 features+=", left="+(window.screen.width-width)/2;
 else width=window.screen.width;
 features+=", width="+width;
 }
 if (height) {
 if (window.screen.height > height)
 features+=", top="+(window.screen.height-height)/2;
 else height=window.screen.height;
 features+=", height="+height;
 }
 window.open(url,windowname,features);
}