docs: def createResource(self):
          """
          When invoked by L{PrefixURLMixin}, return a L{websharing.SharingIndex}
          for my application.
          """
          pp = ixmantissa.IPublicPage(self.application, None)
          if pp is not None:
              warn(
              "Use the sharing system to provide public pages, not IPublicPage",
              category=DeprecationWarning,
              stacklevel=2)
              return pp.getResource()
          return SharingIndex(self.application.open())