code: def get_values(self, attr_name):
          
          ret = list(self._attr_value_cdist[attr_name].keys()) \
              + list(self._attr_value_counts[attr_name].keys()) \
              + list(self._branches.keys())
          ret = set(ret)
          return ret
docs: Retrieves the unique set of values seen for the given attribute
          at this node.