code: def focus_property(self, prop, direction):
          
          newpos = self.get_selected_mid()
          newpos = direction(newpos)
          while newpos is not None:
              MT = self._tree[newpos]
              if prop(MT):
                  newpos = self._sanitize_position((newpos,))
                  self.body.set_focus(newpos)
                  break
              newpos = direction(newpos)
docs: does a walk in the given direction and focuses the
          first message tree that matches the given property