'Returns an ARFF header as a string.'

def header_section(self):

    s = (('%    Weka    ARFF    file\n' + '%    Generated    automatically    by    NLTK\n') + ('%%    %s\n\n' % time.ctime()))
    s += '@RELATION    rel\n\n'
    for (fname, ftype) in self._features:
        s += ('@ATTRIBUTE    %-30r    %s\n' % (fname, ftype))
    s += ('@ATTRIBUTE    %-30r    {%s}\n' % ('-label-', ','.join(self._labels)))
    return s
