code: def add_timex(self, time_obj):
          
          if self.timex_layer is None:
              self.timex_layer = CtimeExpressions()
              self.root.append(self.timex_layer.get_node())
          self.timex_layer.add_timex(time_obj)
docs: Adds a timex entry to the time layer
          @type time_obj: L{Ctime}
          @param time_obj: time time object