new_contents: from zope.interface import implementer
  
  from .interfaces (
          IAuthSourceService,
          )
  
  @implementer(IAuthSourceService)
  class SessionAuthSource(object):
      """ An authentication source that uses the current session """
  
      vary = ()
      value_key = 'sanity.value'
  
      def __init__(self, context, request):
          self.request = request
          self.session = request.session
  
          return self
  
      def get_value(self):
          return self.session.get(value_key, [None, None])
  
      def headers_remember(self, value):
          self.session[value_key] = value
          return []
  
      def headers_forget(self):
          if value_key in self.session:
              del self.session[value_key]
          return []
  
subject: Add a session based source