code: function (val) {
          var color, mode;
          if (arguments.length > 1) color = arguments[1];
          if (arguments.length > 2) mode = arguments[2];
  
          return {
              is: 'tag',
              val: val,
              color: color,
              mode: mode,
              toString: function(env) {
                  var attr = {};
  
                  _.set(attr, 'value', val.ev(env).toString());
                  if (color) {
                      _.set(attr, 'color', color.ev(env).toString());
                  }
                  if (mode) {
                      _.set(attr, 'mode', mode.ev(env).toString());
                  }
                  return {
                      '_name': 'stop',
                      '_attributes': attr
                  };
              }
          };
      }
docs: Only require val