code: def add_external_reference(self,ext_ref):
          
          #check if the externalreferences sublayer exist for the role, and create it in case
          node_ext_refs = self.node.find('externalReferences')
          ext_refs = None
          if node_ext_refs == None:
              ext_refs = CexternalReferences()
              self.node.append(ext_refs.get_node())
          else:
              ext_refs = CexternalReferences(node_ext_refs)
          
          ext_refs.add_external_reference(ext_ref)
docs: Adds an external reference to the role
          @param ext_ref: the external reference object
          @type ext_ref: L{CexternalReference}