returns the ID of the sentence the given element belongs to.

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)