docs: public static synchronized String rot13( String input ) {
          StringBuffer output = new StringBuffer();
          if ( input != null ) {
              for ( int i = 0; i < input.length(); i++ ) {
                  char inChar = input.charAt( i );
                  if ( ( inChar >= 'A' ) & ( inChar <= 'Z' ) ) {
                      inChar += 13;
                      if ( inChar > 'Z' ) {
                          inChar -= 26;
                      }
                  }
                  if ( ( inChar >= 'a' ) & ( inChar <= 'z' ) ) {
                      inChar += 13;
                      if ( inChar > 'z' ) {
                          inChar -= 26;
                      }
                  }
                  output.append( inChar );
              }
          }
          return output.toString();
      }