code: def add_markable(self,markable_obj):
          
          if self.markable_layer is None:
              self.markable_layer = Cmarkables(type=self.type)
              self.root.append(self.markable_layer.get_node())
          self.markable_layer.add_markable(markable_obj)
docs: Adds a markable to the markable layer
          @type markable_obj: L{Cmarkable}
          @param markable_obj: the markable object