docs: def get_sentence_id(self, element):
          """returns the ID of the sentence the given element belongs to."""
          try:
              sentence_elem = element.iterancestors('sentence').next()
          except StopIteration as e:
              warnings.warn("<{}> element is not a descendant of a <sentence> "
                            "We'll try to extract the sentence ID from the "
                            "prefix of the element ID".format(element.tag))
              return self.get_element_id(element).split('_')[0]
          return self.get_element_id(sentence_elem)