code: def get_sentences(self, root_element, block_tags):
          
          sentences = []
          for element in root_element:
              if not self.any_ends_with(block_tags, element.tag):
                  # tag not in block_tags
                  if element.text is not None and not re.match('^\s*$',
                                                               element.text):
                      sentences.extend(self.sentence_tokenize(element.text))
                  sentences.extend(self.get_sentences(element, block_tags))
  
          f = open('sentence_debug.txt', 'w')
          for s in sentences:
              f.write(s.lower() + '\n')
          f.close()
          return sentences
docs: Returns a list of plain-text sentences by iterating through
          XML tags except for those listed in block_tags.