new_contents: class Undercore(object):
      doc_extensions = {}
      span_extensions = {}
      token_extensions = {}
  
      def __init__(self, obj, start=None, end=None):
          object.__setattr__(self, '_obj', obj)
          # Assumption is that for doc values, _start and _end will both be None
          # Span will set non-None values for _start and _end
          # Token will have _start be non-None, _end be None
          # This lets us key everything into the doc.user_data dictionary,
          # (see _get_key), and lets us use a single Underscore class.
          object.__setattr__(self, '_doc', obj.doc)
          object.__setattr__(self, '_start', start)
          object.__setattr__(self, '_end', start)
  
      def __getattr__(self, name):
          if name not in self.__class__.extensions:
              raise AttributeError(name)
          default, method, getter, setter = self.__class__.extensions[name]
          if getter is not None:
              return getter(self._obj)
          elif method is not None:
              return method)
          else:
              return self._doc.user_data.get(self._get_key(name), default)
  
      def __setattr__(self, name, value):
          if name not in self.__class__.extensions:
              raise AttributeError(name)
          default, method, getter, setter = self.__class__.extensions[name]
          if setter is not None:
              return setter(self._obj, value)
          else:
              self._doc.user_data[self._get_key(name)] = value
  
      def _get_key(self, name):
          return ('._.', name, self._start, self._end)
  
subject: Add Underscore class