code: private Expression extract( CssFormatter formatter ) {
          List<Expression> exList = getParamList( formatter );
          int idx = getInt( 1, formatter );
          if( idx <= 0 || exList.size() < idx ) {
              type = STRING;
              return null;
          }
          Expression ex = exList.get( idx - 1 );
          type = ex.getDataType( formatter );
          switch( type ) {
              case STRING:
              case LIST:
                  break;
              default:
                  doubleValue = ex.doubleValue( formatter );
          }
          return ex;
  
      }
docs: Function extract. Change the type and doubleValue
  @param formatter current CSS output
  @return the extracted expression if it is a string