<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def keys(self):
 """Iterates over the hive's keys. Yields WinRegKey namedtuples containing: timestamp: date and time of last modification """

        for node in self.node_children(self.root()):
            yield from self._visit_registry(node, self._rootkey)