<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def config_value_keys(self, sortkey = False):
 """ Return configuration keys directly stored in this node. Configurations in child nodes are not included. """

        if sortkey:
            items = sorted(self.items())
        else:
            items = self.items()
        return (k for k,v in items if not isinstance(v,ConfigTree))