Dragon Singing in a Withered Tree

KOAN:

Xiangyan was once asked by a monastic, “What is the Way?”
Xiangyan said, “A dragon singing in a withered tree.”

Later, another monastic asked Shishuang, “What is the ‘dragon singing in a withered tree?'”
Shishuang said, “It still has joy.”

The monastic also asked Caoshan, “What is the ‘dragon singing in a withered tree?’”
Caoshan said, “The blood line is not cut off.”
The monastic asked, “Who can hear this?”
Caoshan said, “In the whole world, there is no one who does not hear it.”