docs: 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