docs: def add_term(self,term_obj):
          """
          Adds a term object to the layer
          @type term_obj: L{Cterm}
          @param term_obj: the term object
          """
          if term_obj.get_id() in self.idx:
              raise ValueError("Term with id {} already exists!"
                               .format(term_obj.get_id()))
          self.node.append(term_obj.get_node())
          self.idx[term_obj.get_id()] = term_obj