docs: def add_external_reference(self,ext_ref):
          """
          Adds an external reference object to the markable
          @type ext_ref: L{CexternalReference}
          @param ext_ref: an external reference object
          """
          ext_refs_node = self.node.find('externalReferences')
          if ext_refs_node is None:
              ext_refs_obj = CexternalReferences()
              self.node.append(ext_refs_obj.get_node())
          else:
              ext_refs_obj = CexternalReferences(ext_refs_node)
              
          ext_refs_obj.add_external_reference(ext_ref)