code: function ForCommand(value, engine) {
          var rule = new RegExp(
              stringFormat(
                  /* jshint ignore:start */
                  '^\\s*({0}[\\s\\S]+{1})\\s+as\\s+{0}([0-9a-z_]+){1}\\s*(,\\s*{0}([0-9a-z_]+){1})?\\s*$',
                  /* jshint ignore:end */
                  regexpLiteral(engine.options.variableOpen),
                  regexpLiteral(engine.options.variableClose)
              ),
              'i'
          );
  
  
          var match = value.match(rule);
          if (!match) {
              throw new Error('Invalid ' + this.type + ' syntax: ' + value);
          }
  
          this.list = match[1];
          this.item = match[2];
          this.index = match[4];
          Command.call(this, value, engine);
          this.cloneProps = ['list', 'item', 'index'];
      }
docs: for命令节点类
  
  @inner
  @constructor
  @param {string} value 命令节点的value
  @param {Engine} engine 引擎实例