old_contents: from chainer import cuda
  from chainer import function
  from chainer.utils import type_check
  from chainer import variable
  
  
  class _DummyFunction(function.Function):
  
      def __init__(self, grads):
          self.grads = grads
  
      def forward(self, inputs):
          xp = cuda.get_array_module(*inputs)
          return xp.array(0),
  
      def backward(self, inputs, outputs):
          return self.grads
  
  
  class Forget(function.Function):
  
      def __init__(self, func):
          if not callable(func):
              raise TypeError('func must be callable')
  
          self.func = func
  
      def _call_func(self, xs):
          outs = self.func(*xs)
  
          if isinstance(outs, tuple):
              for i, out in enumerate(outs):
                  if isinstance(out, variable.Variable):
                      continue
                  n = i + 1
                  suffix = {1: 'st', 2: 'nd', 3: 'rd'}.get(
                      n if n < 20 else n % 10, 'th')
                  msg = ('{}{} element of a returned tuple is not Variable, '
                         'but is {}').format(n, suffix, type(out))
                  raise RuntimeError(msg)
          elif isinstance(outs, variable.Variable):
              outs = (outs,)
          else:
              msg = ('A tuple of Variables or a Variable are expected, but {} '
                     'is returned.'.format(type(outs)))
              raise RuntimeError(msg)
  
          return outs
  
      def forward(self, inputs):
          xs = [variable.Variable(x, volatile=True) for x in inputs]
          outs = self._call_func(xs)
          return tuple(out.data for out in outs)
  
      def backward(self, inputs, grads):
          xs = [variable.Variable(x, volatile=False) for x in inputs]
          outs = self._call_func(xs)
          _DummyFunction(grads)(*outs).backward()
          return tuple(x.grad for x in xs)
  
  
  def forget(func, *xs):
      return Forget(func)(*xs)
  
new_contents: from chainer import cuda
  from chainer import function
  from chainer import variable
  
  
  class _DummyFunction(function.Function):
  
      def __init__(self, grads):
          self.grads = grads
  
      def forward(self, inputs):
          xp = cuda.get_array_module(*inputs)
          return xp.array(0),
  
      def backward(self, inputs, outputs):
          return self.grads
  
  
  class Forget(function.Function):
  
      def __init__(self, func):
          if not callable(func):
              raise TypeError('func must be callable')
  
          self.func = func
  
      def _call_func(self, xs):
          outs = self.func(*xs)
  
          if isinstance(outs, tuple):
              for i, out in enumerate(outs):
                  if isinstance(out, variable.Variable):
                      continue
                  n = i + 1
                  suffix = {1: 'st', 2: 'nd', 3: 'rd'}.get(
                      n if n < 20 else n % 10, 'th')
                  msg = ('{}{} element of a returned tuple is not Variable, '
                         'but is {}').format(n, suffix, type(out))
                  raise RuntimeError(msg)
          elif isinstance(outs, variable.Variable):
              outs = (outs,)
          else:
              msg = ('A tuple of Variables or a Variable are expected, but {} '
                     'is returned.'.format(type(outs)))
              raise RuntimeError(msg)
  
          return outs
  
      def forward(self, inputs):
          xs = [variable.Variable(x, volatile=True) for x in inputs]
          outs = self._call_func(xs)
          return tuple(out.data for out in outs)
  
      def backward(self, inputs, grads):
          xs = [variable.Variable(x, volatile=False) for x in inputs]
          outs = self._call_func(xs)
          _DummyFunction(grads)(*outs).backward()
          return tuple(x.grad for x in xs)
  
  
  def forget(func, *xs):
      return Forget(func)(*xs)
  
subject: Remove unused import