code: def _merge_key_indices(a, b):
      if a is None and b is None:
          return None
      elif a is None:
          return b
      elif b is None:
          return a
      else:
          return tuple(a[index] for index in b)
  
imports: import numbers
  import numpy
  import six
  from chainer.dataset.tabular import tabular_dataset
label_desc: Use image node_id 4 for calling a global function with example usage: _merge_key_indices(a, b) and returns: None, b, a, tuple