function rollmenu(a){
    var blue = /-b/;
    var pattern = /img\/(.*)\.gif/;
    var img = a.firstChild;
    var img_name = pattern.exec(img.src);
    if(img_name){
        name = img_name[1];
        if(!blue.test(img.src)){
            img.src = "img/"+name+"-b"+".gif";
        }else {
            
            img.src = "img/"+name.substr(0, name.length-2)+".gif";    
        }
    }
    
      
}