code: def connect(self, node, properties=None):
          
          if len(self):
              raise AttemptedCardinalityViolation(
                      "Node already has {0} can't connect more".format(self))
          else:
              return super(ZeroOrOne, self).connect(node, properties)
docs: Connect to a node.
  
          :param node:
          :type: StructuredNode
          :param properties: relationship properties
          :type: dict
          :return: True / rel instance