code: def with_rule(self, rule):
          
          self.rules = self.rules if self.rules != None else []
          self.rules.append(rule)
          return self
docs: Adds validation rule to this schema.
  
          This method returns reference to this exception to implement Builder pattern
          to chain additional calls.
  
          :param rule: a validation rule to be added.
  
          :return: this validation schema.