def count_words(file):
  """ Counts the word frequences in a list of sentences.

  Note:
    This is a helper function for parallel execution of `Vocabulary.from_text`
    method.
  """
  c = Counter()
  with open(file, 'r') as f:
    for l in f:
      words = l.strip().split()
      c.update(words)
  return c