docs: def _getViewerPrivateApplication(self):
          """
          Get the L{PrivateApplication} object for the logged-in user who is
          viewing this resource, as indicated by its C{username} attribute.
  
          This is highly problematic because it precludes the possibility of
          separating the stores of the viewer and the viewee into separate
          processes, and it is only here until we can get rid of it.  The reason
          it remains is that some application code still imports things which
          subclass L{PublicAthenaLivePage} and L{PublicPage} and uses them with
          usernames specified.  See ticket #2702 for progress on this goal.
  
          However, Mantissa itself will no longer set this class's username
          attribute to anything other than None, because authenticated users'
          pages will be generated using
          L{xmantissa.webapp._AuthenticatedWebViewer}.  This method is used only
          to render content in the shell template, and those classes have a direct
          reference to the requisite object.
  
          @rtype: L{PrivateApplication}
          """
          ls = self.store.findUnique(userbase.LoginSystem)
          substore = ls.accountByAddress(*self.username.split('@')).avatars.open()
          from xmantissa.webapp import PrivateApplication
          return substore.findUnique(PrivateApplication)