code: def pos_tag(self):
          
          self._require_tokens()
          self._require_no_ngrams_as_tokens()
  
          self._invalidate_workers_tokens()
          logger.info('POS tagging tokens')
          self._send_task_to_workers('pos_tag')
          self.pos_tagged = True
  
          return self
docs: Apply Part-of-Speech (POS) tagging on each token.
          Uses the default NLTK tagger if no language-specific tagger could be loaded (English is assumed then as
          language). The default NLTK tagger uses Penn Treebank tagset
          (https://ling.upenn.edu/courses/Fall_2003/ling001/penn_treebank_pos.html).
          The default German tagger based on TIGER corpus uses the STTS tagset
          (http://www.ims.uni-stuttgart.de/forschung/ressourcen/lexika/TagSets/stts-table.html).