code: def childFactory(self, ctx, name):
          
          try:
              o = self.webapp.fromWebID(name)
          except _WebIDFormatException:
              return None
          if o is None:
              return None
          return self.webViewer.wrapModel(o)
docs: Return a shell page wrapped around the Item model described by the
          webID, or return None if no such item can be found.