nodejs co generator 调用另外一个generator

co 支持 在yeild 后面的内容（文档中叫做Yieldables）  包括generator

参考文档：https://github.com/tj/co

var co =require("co");function sayHello(userName){    return function(callback)    {        setTimeout(function () {            callback(null,"hello "+userName);        },200)    }}co(function *(){    var a = yield sayHello("a");    console.log(a);    var b = yield sayHello("b");    console.log(b);   yield function * (){       var a = yield sayHello("c");       console.log(a);       var b = yield sayHello("d");       console.log(b);   };}).catch(function(err){    console.log(err.stack);})