public Set<String> attributes() {
        TreeSet<String> attributes = new TreeSet<>();
        Enumeration<String> enumeration = session.getAttributeNames();
        while (enumeration.hasMoreElements()) {
            attributes.add(enumeration.nextElement());
        }
        return attributes;
    }
@return an <code>Enumeration</code> of <code>String</code> objects
containing the names of all the objects bound to this session.
src/main/java/spark/Session.java