code: def switch_zeros_of_values(self):
          
          nb_sons = len(self.sons)
          # Need a list for assignment
          new_values = list(self.of_values)
          for i in [0, 1, 2]:
              if new_values[i] == '0':
                  new_values[i] = str(nb_sons)
          self.of_values = tuple(new_values)
docs: If we are a of: rule, we can get some 0 in of_values,
             if so, change them with NB sons instead
  
          :return: None