code: public function setValue($value)
      {
          if ($value === false) {
              return parent::setValue($value);
          }
          if ($value != $this->getAttribute('value')) {
              return false;
          }
  
          return parent::setValue($value);
      }
docs: The only allowed value sn the one in the "value" attribute.
  
  @param string $value      New value.
  
  @return bool           True if allowed.