docs: def locateChild(self, ctx, segments):
          """
          Retrieve a L{SharingIndex} for a particular user, or rend.NotFound.
          """
          store = _storeFromUsername(
              self.loginSystem.store, segments[0].decode('utf-8'))
          if store is None:
              return rend.NotFound
          return (SharingIndex(store, self.webViewer), segments[1:])