code: def _mean_or_nan(xs):
      """Return its mean a non-empty sequence, numpy.nan for a empty one."""
      return np.mean(xs) if xs else np.nan
  
imports: import collections
  import itertools
  import random
  import chainer
  from chainer import cuda
  import chainer.functions
  import numpy
  from chainerrl import agent
  from chainerrl.misc.batch_states import batch_states
label_desc: Use image node_id 1 for calling a global function with example usage: _mean_or_nan(xs) and returns: unknown