old_contents: from chainer.backends import cuda
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
  
  
  class Arctanh(function_node.FunctionNode):
  
      """Elementwise inverse hyperbolic tangent function."""
  
      def check_type_forward(self, in_types):
          type_check._argname(in_types, ('x',))
          x_type, = in_types
  
          type_check.expect(x_type.dtype.kind == 'f')
  
      def forward(self, inputs):
          self.retain_inputs((0,))
          x, = inputs
          xp = cuda.get_array_module(x)
          y = xp.arctanh(x)
          return utils.force_array(y, dtype=x.dtype),
  
      def backward(self, indexes, grad_outputs):
          x, = self.get_retained_inputs()
          gy, = grad_outputs
          gx = 1. / (1 - x ** 2) * gy
          return gx,
  
  
  def arctanh(x):
      """Elementwise inverse hyperbolic tangent function.
  
      Args:
          x (:class:`~chainer.Variable` or :ref:`ndarray`): Input variable.
  
      Returns:
          ~chainer.Variable: Output variable.
  
      """
      return Arctanh().apply((x,))[0]
  
new_contents: from chainer.backends import cuda
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
  
  
  class Arctanh(function_node.FunctionNode):
  
      """Elementwise inverse hyperbolic tangent function."""
  
      def check_type_forward(self, in_types):
          if hasattr(type_check, '_argname'):
              # typecheck._argname is introduced by Chainer v6
              type_check._argname(in_types, ('x',))
          x_type, = in_types
  
          type_check.expect(x_type.dtype.kind == 'f')
  
      def forward(self, inputs):
          self.retain_inputs((0,))
          x, = inputs
          xp = cuda.get_array_module(x)
          y = xp.arctanh(x)
          return utils.force_array(y, dtype=x.dtype),
  
      def backward(self, indexes, grad_outputs):
          x, = self.get_retained_inputs()
          gy, = grad_outputs
          gx = 1. / (1 - x ** 2) * gy
          return gx,
  
  
  def arctanh(x):
      """Elementwise inverse hyperbolic tangent function.
  
      Args:
          x (:class:`~chainer.Variable` or :ref:`ndarray`): Input variable.
  
      Returns:
          ~chainer.Variable: Output variable.
  
      """
      return Arctanh().apply((x,))[0]
  
subject: Fix chainer v4 error about type_check._argname