code: def _get_combination(combination, tensors):
      if combination.isdigit():
          index = int(combination) - 1
          return tensors[index]
      else:
          if len(combination) != 3:
              raise ConfigurationError(
                  "Invalid combination: " + combination
              )
          first_tensor = _get_combination(combination[0], tensors)
          second_tensor = _get_combination(combination[2], tensors)
          operation = combination[1]
          if operation == "*":
              return first_tensor * second_tensor
          elif operation == "/":
              return first_tensor / second_tensor
          elif operation == "+":
              return first_tensor + second_tensor
          elif operation == "-":
              return first_tensor - second_tensor
          else:
              raise ConfigurationError(
                  "Invalid operation: " + operation
              )
  
imports: from __future__ import division
  from __future__ import absolute_import
  from collections import defaultdict
  import logging
  import math
  import torch
  from allennlp.common.checks import ConfigurationError
label_desc: Use image node_id 21 for calling a global function with example usage: _get_combination(combination, tensors) and returns: tensors, unknown, unknown, unknown, unknown