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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def unused_keys(self):
 """Lists all keys which are present in the ConfigTree but which have not been accessed."""

        unused = set()
        for k, c in self._children.items():
            if isinstance(c, ConfigNode):
                if not c.has_been_accessed():
                    unused.add(k)
            else:
                for ck in c.unused_keys():
                    unused.add(k+'.'+ck)
        return unused