The option definition

gistfile1.php

<?php
  /**
   * Return all possible options for the view and provide default values.
   */
  function option_definition() {
    $options = parent::option_definition();
    $options['flags'] = array('default' => array());
    $options['vocabularies'] = array('default' => array());
    $options['fallback'] = array('default' => 'all');
    $options['multiple_operator'] = array('default' => '+');
    return $options;
  }