code: public int decide(final LoggingEvent event) {
      if (expressionRule.evaluate(event, null)) {
            event.getLocationInformation();
      }
      return Filter.NEUTRAL;
    }
docs: If this event does not already contain location information,
  evaluate the event against the expression.
  
  If the expression evaluates to true,
  force generation of location information by
  calling getLocationInfo.
  
  @param event event
  @return Filter.NEUTRAL.