code: def ensure_security_manager(self, subject_context):
          
          if (subject_context.resolve_security_manager() is not None):
              msg = ("Subject Context resolved a security_manager "
                     "instance, so not re-assigning.  Returning.")
              logger.debug(msg)
              return subject_context
  
          msg = ("No security_manager found in context.  Adding self "
                 "reference.")
          logger.debug(msg)
  
          subject_context.security_manager = self
  
          return subject_context
docs: Determines whether there is a ``SecurityManager`` instance in the context,
          and if not, adds 'self' to the context.  This ensures that do_create_subject
          will have access to a ``SecurityManager`` during Subject construction.
  
          :param subject_context: the subject context data that may contain a
                                  SecurityManager instance
          :returns: the SubjectContext