old_contents: from chainer.functions.array import concat
  from chainer.functions.array import expand_dims
  
  
  def stack(xs, axis=0):
      xs = [expand_dims.expand_dims(x, axis=axis) for x in xs]
      return concat.concat(xs, axis=axis)
  
new_contents: from chainer.functions.array import concat
  from chainer.functions.array import expand_dims
  
  
  def stack(xs, axis=0):
      """Concatenate variables along a new axis.
  
      Args:
          xs (list of chainer.Variable): Variables to be concatenated.
          axis (int): Axis of result along which variables stacked.
  
      Returns:
          ~chainer.Variable: Output variable.
  
      """
      xs = [expand_dims.expand_dims(x, axis=axis) for x in xs]
      return concat.concat(xs, axis=axis)
  
subject: Fix docstring