def from_conll(this_class, stream):
        """Construct a Corpus. stream is an iterable over strings where
        each string is a line in CoNLL-X format."""
        stream = iter(stream)
        corpus = this_class()
        while 1:
            # read until we get an empty sentence
            sentence = Sentence.from_conll(stream)
            if sentence:
                corpus.append(sentence)
            else:
                break
        return corpus
Construct a Corpus. stream is an iterable over strings where
        each string is a line in CoNLL-X format.