docs: def score(count_bigram, count1, count2, n_words):
      """Collocation score"""
      if n_words <= count1 or n_words <= count2:
          # only one words appears in the whole document
          return 0
      N = n_words
      c12 = count_bigram
      c1 = count1
      c2 = count2
      p = c2 / N
      p1 = c12 / c1
      p2 = (c2 - c12) / (N - c1)
      score = (l(c12, c1, p) + l(c2 - c12, N - c1, p)
               - l(c12, c1, p1) - l(c2 - c12, N - c1, p2))
      return -2 * score