create not-yet-evaluated object在R中创建 not-yet-evaluated(就是some code we will evaluated later!!)对象;然后执行的时候需要调用eval(参数中提供执行环境)或者do.ca()执行；
do.call()作用就是
do.call( "someRFunction",aListOfArgs)
<=>
someRFuncion( aListOfArgs );
do.call() is constructing a function call from the list of arguments