code: def installed(cls):
          
          try:
              return cls._installed
          except AttributeError:
              name = "yacms.pages.middleware.PageMiddleware"
              mw_setting = get_middleware_setting()
              installed = name in mw_setting
              if not installed:
                  for name in mw_setting:
                      if issubclass(import_dotted_path(name), cls):
                          installed = True
                          break
              setattr(cls, "_installed", installed)
              return installed
docs: Used in ``yacms.pages.views.page`` to ensure
          ``PageMiddleware`` or a subclass has been installed. We cache
          the result on the ``PageMiddleware._installed`` to only run
          this once. Short path is to just check for the dotted path to
          ``PageMiddleware`` in ``MIDDLEWARE_CLASSES`` - if not found,
          we need to load each middleware class to match a subclass.