How Do You Cross a Stream? (Record of Dongshan)

KOAN:

Shenshan was crossing a river with his dharma brother Dongshan. Dongshan said, “Don’t make a mistake with your steps and slip into the current.”
Shenshan said, “If I make a mistake with my steps, then I won’t live to cross the river.”
Dongshan said, “What is the state without mistakes?”
Shenshan said, “Now I’m crossing the river with the Ancestor.”