code: def single(self):
          
          nodes = super(One, self).all()
          if nodes:
              if len(nodes) == 1:
                  return nodes[0]
              else:
                  raise CardinalityViolation(self, len(nodes))
          else:
              raise CardinalityViolation(self, 'none')
docs: Return the associated node.
  
          :return: node