code: def _unpack_index(i):
      """Unpack index and return exactly four elements.
  
      If index is more shallow than 4, return None for trailing
      dimensions. If index is deeper than 4, raise a KeyError.
  
      """
      if len(i) > 4:
          raise KeyError(
              "Tried to index history with {} indices but only "
              "4 indices are possible.".format(len(i))
          )
  
      # fill trailing indices with None
      i_e, k_e, i_b, k_b = i + tuple([None] * (4 - len(i)))
  
      return i_e, k_e, i_b, k_b
  
imports: import json
  import pickle
  from skorch.utils import open_file_like
label_desc: Use image node_id 9 for calling a global function with example usage: _unpack_index(i) and returns: i_e,  k_e,  i_b,  k_b