def single(self):
nodes = super(ZeroOrOne, self).all()<EOL>if len(nodes) == <NUM_LIT:1>:<EOL><INDENT>return nodes[<NUM_LIT:0>]<EOL><DEDENT>if len(nodes) > <NUM_LIT:1>:<EOL><INDENT>raise CardinalityViolation(self, len(nodes))<EOL><DEDENT>
Return the associated node.

:return: node