code: private void evalParam( int idx, CssFormatter formatter ) {
          Expression expr = get( idx );
          type = expr.getDataType( formatter );
          switch( type ) {
              case BOOLEAN:
                  booleanValue = expr.booleanValue( formatter );
                  break;
              case STRING:
                  break;
              default:
                  doubleValue = expr.doubleValue( formatter );
          }
      }
docs: Evaluate a parameter as this function.
  
  @param idx
  the index of the parameter starting with 0
  @param formatter
  the current formation context