code: @Override
      public String unit( CssFormatter formatter ) {
          switch( super.toString() ) {
              case "unit":
                  if( parameters.size() <= 1 ) {
                      return "";
                  } else {
                      return get( 1 ).stringValue( formatter );
                  }
              case "convert":
                  return get( 1 ).stringValue( formatter );
              case "sin":
              case "cos":
              case "tan":
              case "length":
                  return "";
              case "acos":
              case "asin":
              case "atan":
                  return "rad";
              case "pow":
                  return get( 0 ).unit( formatter );
              case "alpha":
              case "red":
              case "green":
              case "blue":
              case "rgba":
              case "rgb":
              case "argb":
              case "saturate":
              case "desaturate":
              case "greyscale":
              case "hsl":
              case "hsla":
              case "hue":
              case "spin":
              case "lighten":
              case "darken":
              case "fadein":
              case "fadeout":
              case "fade":
              case "hsv":
              case "hsva":
              case "hsvhue":
              case "contrast":
                  // color values has no unit
                  return "";
              case "saturation":
              case "lightness":
              case "hsvsaturation":
              case "hsvvalue":
              case "luma":
              case "luminance":
                  return "%";
              case "if":
                  return get( 1 ).unit( formatter );
          }
          for( int i = 0; i < parameters.size(); i++ ) {
              String unit = parameters.get( i ).unit( formatter );
              if( !unit.isEmpty() ) {
                  return unit;
              }
          }
          return "";
      }
docs: {@inheritDoc}