code: def get_nth_unique_value(self, keypath, n, distance_from, open_interval=True):
          
          unique_values = self.get_ordered_values(keypath, distance_from, open_interval)
          if 0 <= n < len(unique_values):
              #logging.error("%i th unique value is %s" % (n, str(unique_values[n])))
              return unique_values[n]
          else:
              raise Contradiction("n-th Unique value out of range: " + str(n))
docs: Returns the `n-1`th unique value, or raises
          a contradiction if that is out of bounds