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, Bool, List, Instance
  
  #-----------------------------------------------------------------------------
  # Classes
  #-----------------------------------------------------------------------------
  class ContainerWidget(DOMWidget):
      view_name = Unicode('ContainerView', sync=True)
  
      # Keys, all private and managed by helper methods.  Flexible box model
      # classes...
      children = List(Instance(DOMWidget))
      _children = List(Instance(DOMWidget), sync=True)
  
      def __init__(self, *pargs, **kwargs):
          """Constructor"""
          DOMWidget.__init__(self, *pargs, **kwargs)
          self.on_trait_change(self._validate, ['children'])
  
      def _validate(self, name, old, new):
          """Validate children list.
  
          Makes sure only one instance of any given model can exist in the 
          children list."""
          if new is not None and isinstance(new, list):
              children = []
              for child in new:
                  if child not in children:
                      children.append(child)
              self._children = children
  
  
  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, Bool, List, Instance
  
  #-----------------------------------------------------------------------------
  # Classes
  #-----------------------------------------------------------------------------
  class ContainerWidget(DOMWidget):
      view_name = Unicode('ContainerView', sync=True)
  
      # Keys, all private and managed by helper methods.  Flexible box model
      # classes...
      children = List(Instance(DOMWidget))
      _children = List(Instance(DOMWidget), sync=True)
  
      def __init__(self, *pargs, **kwargs):
          """Constructor"""
          DOMWidget.__init__(self, *pargs, **kwargs)
          self.on_trait_change(self._validate, ['children'])
  
      def _validate(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
              return [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: Replace O(N^2) algorithm with a faster one.