private void testExistingField(String name, String descriptor)
            throws DuplicateMemberException {
        ListIterator it = fields.listIterator(0);
        while (it.hasNext()) {
            FieldInfo minfo = (FieldInfo)it.next();
            if (minfo.getName().equals(name))
                throw new DuplicateMemberException("duplicate field: " + name);
        }
    }