class_docstr: Base class for BLEU & NIST, providing tokenization and some basic n-gram matching
  functions.
docstr: Add a sentence to the statistics.
  @param pred_sent: system output / predicted sentence
  @param ref_sents: reference sentences
docstr_tok: ["Add","a","sentence","to","the","statistics",".","@","param","pred_sent",":","system","output","\/","predicted","sentence","@","param","ref_sents",":","reference","sentences"]
code: def append(self, pred_sent, ref_sents):
      """Add a sentence to the statistics.
      @param pred_sent: system output / predicted sentence
      @param ref_sents: reference sentences
      """
      raise NotImplementedError()
  
imports: from __future__ import unicode_literals
  from __future__ import division
  from builtins import zip
  from builtins import range
  from past.utils import old_div
  from builtins import object
  from collections import defaultdict
  import math
  import re
label_desc: Use image node_id 3 for calling the NGramScore obj's underlying member method code with example usage: obj.append(pred_sent, ref_sents) without return types