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

varco=require("co");functionsayHello(userName){returnfunction(callback){setTimeout(function(){callback(null,"hello"+userName);},200)}}co(function*(){vara=yieldsayHello("a");console.log(a);varb=yieldsayHello("b");console.log(b);yieldfunction*(){vara=yieldsayHello("c");console.log(a);varb=yieldsayHello("d");console.log(b);};}).catch(function(err){console.log(err.stack);})