old_contents: import pkg_resources
  
  from chainer import flag
  from chainer import function
  from chainer import function_set
  from chainer.functions import basic_math
  from chainer import link
  from chainer import optimizer
  from chainer import serializer
  from chainer import variable
  
  
  __version__ = pkg_resources.get_distribution('chainer').version
  
  AbstractSerializer = serializer.AbstractSerializer
  Chain = link.Chain
  ChainList = link.ChainList
  Deserializer = serializer.Deserializer
  Flag = flag.Flag
  Function = function.Function
  FunctionSet = function_set.FunctionSet
  GradientMethod = optimizer.GradientMethod
  Link = link.Link
  Optimizer = optimizer.Optimizer
  Serializer = serializer.Serializer
  Variable = variable.Variable
  
  ON = flag.ON
  OFF = flag.OFF
  AUTO = flag.AUTO
  
  _debug = False
  
  
  def get_debug():
      return _debug
  
  
  def set_debug(debug):
      global _debug
      _debug = debug
  
  
  basic_math.install_variable_arithmetics()
  
new_contents: import pkg_resources
  
  from chainer import flag
  from chainer import function
  from chainer import function_set
  from chainer.functions import basic_math
  from chainer import link
  from chainer import optimizer
  from chainer import serializer
  from chainer import variable
  
  
  __version__ = pkg_resources.get_distribution('chainer').version
  
  AbstractSerializer = serializer.AbstractSerializer
  Chain = link.Chain
  ChainList = link.ChainList
  Deserializer = serializer.Deserializer
  Flag = flag.Flag
  Function = function.Function
  FunctionSet = function_set.FunctionSet
  GradientMethod = optimizer.GradientMethod
  Link = link.Link
  Optimizer = optimizer.Optimizer
  Serializer = serializer.Serializer
  Variable = variable.Variable
  
  ON = flag.ON
  OFF = flag.OFF
  AUTO = flag.AUTO
  
  _debug = False
  
  
  def get_debug():
      """Get the debug mode.
  
      Returns:
          bool: Return True if Chainer is in debug mode.
      """
      return _debug
  
  
  def set_debug(debug):
      """Set the debug mode.
  
      Args:
          debug (bool): New debug mode.
      """
      global _debug
      _debug = debug
  
  
  basic_math.install_variable_arithmetics()
  
subject: Write docstring