code: static String replacePlaceHolder( CssFormatter formatter, String str, LessObject caller ) {
          int pos = str.startsWith( "@{" ) ? 0 : str.indexOf( "@", 1 );
          if( pos >= 0 ) {
              formatter.addOutput();
              SelectorUtils.appendToWithPlaceHolder( formatter, str, pos, false, caller );
              return formatter.releaseOutput();
          }
          return str;
      }
docs: Replace the possible variable place holder.
  @param formatter current formatter
  @param str the string
  @param caller for exception handling
  @return the result