code: public function setMultiSelect($name, $value, $default = false)
      {
          if (isset($this->data[$name])) {
              if (in_array($value, $this->data[$name])) {
                  return 'selected';
              }
              else {
                  return '';
              }
          }
          else {
              if ($default) {
                  return 'selected';
              }
              else {
                  return '';
              }
          }
      }
docs: For resetting a form Select's data after a failed validation.
  $name = Select's name
  $value = Option's value
  $default = whether this Option should be selected by default.