<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def setnode(delta, graph, node, exists):
 """Change a delta to say that a node was created or deleted"""

    delta.setdefault(graph, {}).setdefault('nodes', {})[node] = bool(exists)