code: def perplexity(items):
      return math.exp(-mean(items))
  
imports: import math
  from collections.abc import Iterable
  import numpy
  import sacrebleu
  import sklearn.metrics
  import random
  from collections import defaultdict
label_desc: Use image node_id 13 for calling a global function with example usage: perplexity(items) and returns: math