'Return a list of file identifiers for the files that make up
this corpus, or that store the given document(s) if specified.'

def fileids(self, documents=None):

    if (documents is None):
        return self._fileids
    elif isinstance(documents, string_types):
        documents = [documents]
    return sorted(set(([('%s.pos' % doc) for doc in documents] + [('%s.psd' % doc) for doc in documents])))
