function popup(url,ancho,alto,id,extras){
	if(navigator.userAgent.indexOf("Mac")>0){ancho=ancho+15;alto=alto+15;}
	var left = (screen.availWidth-ancho)/2;
	var top = (screen.availHeight-alto)/2;
	if(extras!=""){extras=","+extras;};
	eval("window.open('"+url+"','"+id+"','width="+ancho+",height="+alto+",left="+left+",top="+top+",screenX="+left+",screenY="+top+extras+"')");
}