old_contents: """ContainerWidget class.  
  
  Represents a container that can be used to group other widgets.
  """
  #-----------------------------------------------------------------------------
  # Copyright (c) 2013, the IPython Development Team.
  #
  # Distributed under the terms of the Modified BSD License.
  #
  # The full license is in the file COPYING.txt, distributed with this software.
  #-----------------------------------------------------------------------------
  
  #-----------------------------------------------------------------------------
  # Imports
  #-----------------------------------------------------------------------------
  from .widget import DOMWidget
  from IPython.utils.traitlets import Unicode, Tuple, Instance, TraitError
  
  #-----------------------------------------------------------------------------
  # Classes
  #-----------------------------------------------------------------------------
  
  class TupleOfDOMWidgets(Tuple):
      """Like Tuple(Instance(DOMWidget)), but without checking length."""
      def validate_elements(self, obj, value):
          for v in value:
              if not isinstance(v, DOMWidget):
                  raise TraitError("Container.children must be DOMWidgets, not %r" % v)
          return value
  
  class ContainerWidget(DOMWidget):
      _view_name = Unicode('ContainerView', sync=True)
  
      # Keys, all private and managed by helper methods.  Flexible box model
      # classes...
      children = TupleOfDOMWidgets()
      _children = TupleOfDOMWidgets(sync=True)
  
      def _children_changed(self, name, old, new):
          """Validate children list.
  
          Makes sure only one instance of any given model can exist in the 
          children list.
          An excellent post on uniqifiers is available at 
              http://www.peterbe.com/plog/uniqifiers-benchmark
          which provides the inspiration for using this implementation.  Below
          I've implemented the `f5` algorithm using Python comprehensions."""
          if new is not None and isinstance(new, list):
              seen = {}
              def add_item(i):
                  seen[i.model_id] = True
                  return i
              self._children = [add_item(i) for i in new if not i.model_id in seen]
  
  
  class PopupWidget(ContainerWidget):
      _view_name = Unicode('PopupView', sync=True)
      
      description = Unicode(sync=True)
      button_text = Unicode(sync=True)
  
new_contents: """ContainerWidget class.  
  
  Represents a container that can be used to group other widgets.
  """
  #-----------------------------------------------------------------------------
  # Copyright (c) 2013, the IPython Development Team.
  #
  # Distributed under the terms of the Modified BSD License.
  #
  # The full license is in the file COPYING.txt, distributed with this software.
  #-----------------------------------------------------------------------------
  
  #-----------------------------------------------------------------------------
  # Imports
  #-----------------------------------------------------------------------------
  from .widget import DOMWidget
  from IPython.utils.traitlets import Unicode, Tuple, Instance, TraitError
  
  #-----------------------------------------------------------------------------
  # Classes
  #-----------------------------------------------------------------------------
  
  class TupleOfDOMWidgets(Tuple):
      """Like Tuple(Instance(DOMWidget)), but without checking length."""
      def validate_elements(self, obj, value):
          for v in value:
              if not isinstance(v, DOMWidget):
                  raise TraitError("Container.children must be DOMWidgets, not %r" % v)
          return value
  
  class ContainerWidget(DOMWidget):
      _view_name = Unicode('ContainerView', sync=True)
  
      # Keys, all private and managed by helper methods.  Flexible box model
      # classes...
      children = TupleOfDOMWidgets()
      _children = TupleOfDOMWidgets(sync=True)
  
      def _children_changed(self, name, old, new):
          """Validate children list.
  
          Makes sure only one instance of any given model can exist in the 
          children list.
          An excellent post on uniqifiers is available at 
              http://www.peterbe.com/plog/uniqifiers-benchmark
          which provides the inspiration for using this implementation.  Below
          I've implemented the `f5` algorithm using Python comprehensions."""
          if new is not None:
              seen = {}
              def add_item(i):
                  seen[i.model_id] = True
                  return i
              self._children = [add_item(i) for i in new if not i.model_id in seen]
  
  
  class PopupWidget(ContainerWidget):
      _view_name = Unicode('PopupView', sync=True)
      
      description = Unicode(sync=True)
      button_text = Unicode(sync=True)
  
subject: remove incorrect is instance check in children_changed