code: def unused_keys(self):
          
          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
docs: Lists all keys which are present in the ConfigTree but which have not been accessed.