/**
  * Returns and/or create the single instance of this class.  
  *
  * @return  Charitable_Licenses_Settings
  * @access  public
  * @since   1.2.0
  */
 public static function get_instance()
 {
     if (is_null(self::$instance)) {
         self::$instance = new Charitable_Licenses_Settings();
     }
     return self::$instance;
 }
 *
 * @see     Charitable_Gateway_Settings::add_gateway_fields()
 */
add_filter('charitable_settings_tab_fields_gateways', array(Charitable_Gateway_Settings::get_instance(), 'add_gateway_fields'), 5);
/**
 * Add settings to the Email tab.
 *
 * @see     Charitable_Email_Settings::add_email_fields()
 */
add_filter('charitable_settings_tab_fields_emails', array(Charitable_Email_Settings::get_instance(), 'add_email_fields'), 5);
/**
 * Add settings for the Licenses tab.
 *
 * @see     Charitable_Licenses_Settings::add_licenses_fields()
 */
add_filter('charitable_settings_tab_fields_licenses', array(Charitable_Licenses_Settings::get_instance(), 'add_licenses_fields'), 5);
/**
 * Add settings to the Advanced tab.
 *
 * @see     Charitable_Advanced_Settings::add_advanced_fields()
 */
add_filter('charitable_settings_tab_fields_advanced', array(Charitable_Advanced_Settings::get_instance(), 'add_advanced_fields'), 5);
/**
 * Add extra settings for the individual gateways & emails tabs.
 *
 * @see     Charitable_Gateway_Settings::add_individual_gateway_fields()
 * @see     Charitable_Email_Settings::add_individual_email_fields()
 */
add_filter('charitable_settings_tab_fields', array(Charitable_Gateway_Settings::get_instance(), 'add_individual_gateway_fields'), 5);
add_filter('charitable_settings_tab_fields', array(Charitable_Email_Settings::get_instance(), 'add_individual_email_fields'), 5);
/**