code: def check_backward(self, x_data, y_data, *gz_data):
      def f(x, y):
          if self.out_len == 1:
              return functions.forget(lambda x, y: (x + y + x), x, y)
          elif self.out_len == 2:
              return functions.forget(
                  lambda x, y: (x + y + x, x * y), x, y
              )
  
      gradient_check.check_backward(
          f, (x_data, y_data), gz_data, **self.check_backward_options
      )
  
imports: import functools
  import unittest
  import numpy
  import six
  import chainer
  from chainer import cuda
  from chainer import functions
  from chainer import gradient_check
  from chainer import links
  from chainer import testing
  from chainer.testing import attr
  from chainer import variable
label_desc: Use image node_id 4 for calling the TestForget obj's underlying member method code with example usage: obj.check_backward(x_data, y_data) and returns: functions, functions