code: def add_property(self,label,term_span,pid=None):
          
          if self.features_layer is None:
              self.features_layer = Cfeatures(type=self.type)
              self.root.append(self.features_layer.get_node())
          self.features_layer.add_property(pid, label,term_span)
docs: Adds a property to the property layer
          @type label: string
          @param label: the type of property
          @type term_span: list
          @param term_span: list of term ids
          @type pid: string
          @param pid: the identifier for the property (use None to automatically generate one)