old_contents: # http://code.activestate.com/recipes/577413-topological-sort/
  #
  # The MIT License (MIT)
  #
  # Copyright (c) ActiveState.com
  #
  # Permission is hereby granted, free of charge, to any person obtaining a copy
  # of this software and associated documentation files (the "Software"), to deal
  # in the Software without restriction, including without limitation the rights
  # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  # copies of the Software, and to permit persons to whom the Software is
  # furnished to do so, subject to the following conditions:
  #
  # The above copyright notice and this permission notice shall be included in
  # all copies or substantial portions of the Software.
  #
  # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  # THE SOFTWARE.
  
  try:
      from functools import reduce
  except:
      pass
  
  def toposort2(data):
      for k, v in data.items():
          v.discard(k) # Ignore self dependencies
  
      # add items that are listed as dependencies but not as dependents to data
      extra_items_in_deps = reduce(set.union, data.values()) - set(data.keys())
      data.update({item:set() for item in extra_items_in_deps})
  
      while True:
          ordered = set(item for item,dep in data.items() if len(dep) == 0)
          if len(ordered) == 0:
              break
          yield sorted(ordered)
          data = {item: (dep - ordered) for item,dep in data.items()
                                                         if item not in ordered}
  
      assert not data, "A cyclic dependency exists amongst %r" % data
  
new_contents: # http://code.activestate.com/recipes/577413-topological-sort/
  #
  # The MIT License (MIT)
  #
  # Copyright (c) ActiveState.com
  #
  # Permission is hereby granted, free of charge, to any person obtaining a copy
  # of this software and associated documentation files (the "Software"), to deal
  # in the Software without restriction, including without limitation the rights
  # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  # copies of the Software, and to permit persons to whom the Software is
  # furnished to do so, subject to the following conditions:
  #
  # The above copyright notice and this permission notice shall be included in
  # all copies or substantial portions of the Software.
  #
  # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  # THE SOFTWARE.
  
  try:
      from functools import reduce
  except:
      pass
  
  def toposort2(data):
      for k, v in data.items():
          v.discard(k) # Ignore self dependencies
  
      # add items that are listed as dependencies but not as dependents to data
      extra_items_in_deps = reduce(set.union, data.values()) - set(data.keys())
      data.update(dict([(item,set()) for item in extra_items_in_deps]))
  
      while True:
          ordered = set(item for item,dep in data.items() if len(dep) == 0)
          if len(ordered) == 0:
              break
          yield sorted(ordered)
          data = dict([(item, (dep - ordered)) for item,dep in data.items()
                                                         if item not in ordered])
  
      assert not data, "A cyclic dependency exists amongst %r" % data
  
subject: replace dict comprehension in toposort for python 2.6