anchor.diff:  
     for (final file in files) {
       final document = xml.parse(file.readAsStringSync());
  -    stdout.writeln(document.toXmlString(pretty: true, indent: indent));
  +    stdout.writeln(document.toXmlString(pretty: pretty, indent: indent));
     }
   }
  
positive.code:   ..addOption('indent',
        abbr: 'i',
        help: 'Customizes the indention when pretty printing.',
        defaultsTo: '  ');
  
  void printUsage() {
    stdout.writeln('Usage: xml_pp [options] {files}');
  
negative.code:   final files = <File>[];
    final results = argumentParser.parse(arguments);
    final indent = results['indent'];
    final pretty = results['pretty'];
  
    for (final argument in results.rest) {
      final file = File(argument);
  