code: def _get_word_ngrams(n, sentences):
      assert len(sentences) > 0
      assert n > 0
  
      words = set()
      for sentence in sentences:
          words.update(_get_ngrams(n, _split_into_words([sentence])))
  
      return words
  
file_path: sumy/sumy/evaluation/rouge.py
imports: from __future__ import absolute_import
  from __future__ import division, print_function, unicode_literals
  from ..models.dom import Sentence
label_desc: Use image node_id 3 for calling a global function with example usage: _get_word_ngrams(n, sentences) and returns: words