old_contents: """
  meta.py
  
  Some useful metaclasses.
  """
  
  from __future__ import unicode_literals
  
  class LeafClassesMeta(type):
  	"""
  	A metaclass for classes that keeps track of all of them that
  	aren't base classes.
  	"""
  
  	_leaf_classes = set()
  
  	def __init__(cls, name, bases, attrs):
  		if not hasattr(cls, '_leaf_classes'):
  			cls._leaf_classes = set()
  		leaf_classes = getattr(cls, '_leaf_classes')
  		leaf_classes.add(cls)
  		# remove any base classes
  		leaf_classes -= set(bases)
  
  
  class TagRegistered(type):
  	"""
  	As classes of this metaclass are created, they keep a registry in the
  	base class of all classes by a class attribute, 'tag'.
  	"""
  	def __init__(cls, name, bases, namespace):
  		super(TagRegistered, cls).__init__(name, bases, namespace)
  		if not hasattr(cls, '_registry'):
  			cls._registry = {}
  		attr = getattr(cls, 'tag', None)
  		if attr:
  			cls._registry[attr] = cls
  
new_contents: """
  meta.py
  
  Some useful metaclasses.
  """
  
  from __future__ import unicode_literals
  
  class LeafClassesMeta(type):
  	"""
  	A metaclass for classes that keeps track of all of them that
  	aren't base classes.
  	"""
  
  	_leaf_classes = set()
  
  	def __init__(cls, name, bases, attrs):
  		if not hasattr(cls, '_leaf_classes'):
  			cls._leaf_classes = set()
  		leaf_classes = getattr(cls, '_leaf_classes')
  		leaf_classes.add(cls)
  		# remove any base classes
  		leaf_classes -= set(bases)
  
  
  class TagRegistered(type):
  	"""
  	As classes of this metaclass are created, they keep a registry in the
  	base class of all classes by a class attribute, indicated by attr_name.
  	"""
  	attr_name = 'tag'
  
  	def __init__(cls, name, bases, namespace):
  		super(TagRegistered, cls).__init__(name, bases, namespace)
  		if not hasattr(cls, '_registry'):
  			cls._registry = {}
  		meta = cls.__class__
  		attr = getattr(cls, meta.attr_name, None)
  		if attr:
  			cls._registry[attr] = cls
  
subject: Allow attribute to be customized in TagRegistered