uglify2之TreeTransformer

        node.DEFMETHOD("transform", function(tw, in_list){
            var x, y;
            tw.push(this);
            if (tw.before) x = tw.before(this, descend, in_list);
            if (x === undefined) {
                if (!tw.after) {
                    x = this;
                    descend(x, tw);
                } else {
                    tw.stack[tw.stack.length - 1] = x = this.clone();
                    descend(x, tw);
                    y = tw.after(x, in_list);
                    if (y !== undefined) x = y;
                }
            }
            tw.pop();
            return x;
        });