code: Expression getValue( CssFormatter formatter ) {
          String name = toString();
          Expression value = formatter.getVariable( name );
          if( value != null ) {
              return value;
          }
          if( name.startsWith( "@@" ) ) {
              name = name.substring( 1 );
              value = formatter.getVariable( name );
              if( value != null ) {
                  formatter.setInlineMode( true );
                  name = '@' + value.stringValue( formatter );
                  formatter.setInlineMode( false );
                  value = formatter.getVariable( name );
                  if( value != null ) {
                      return value;
                  }
              }
          }
          throw createException( "Undefined Variable: " + name );
      }
docs: Get the referencing expression
  @param formatter current formatter with all variables
  @return the Expression