'Return a set of all keywords used in the corpus.
:param fileids: a list or regexp specifying the ids of the files whose
keywords have to be returned.
:return: the set of keywords and comparative phrases used in the corpus.
:rtype: set(str)'

def keywords(self, fileids=None):

    all_keywords = concat([self.CorpusView(path, self._read_keyword_block, encoding=enc) for (path, enc, fileid) in self.abspaths(fileids, True, True)])
    keywords_set = set([keyword.lower() for keyword in all_keywords if keyword])
    return keywords_set
