帮帮忙　斑竹，解释下下面那段代码：
function alai_tree_check()
{
if(typeof(alai_tree)!="function")
{
alert("run alai_tree_check() fail, please load alai_tree firt!")
return
}
//add(toNode,relation,text,key,ico,exeCategory,exeArg)
var colChkNode=[]
alai_tree.prototype.colChkNode=colChkNode
alai_tree.prototype.addChkNode=function(toNode,text,ico,value,exeArg,checked)
{
var newNode=this.add(toNode,"last",text,"",ico,value,exeArg)
var chkBox=document.createElement('<input type="Checkbox" >')
var tree=this
newNode.label.insertAdjacentElement("beforeBegin",chkBox)
newNode.isCheck=true
if(typeof(checked)=="boolean")chkBox.checked=checked;
newNode.oncheck=new Function("return true;")
chkBox.onpropertychange=function(){if(newNode.oncheck())tree.oncheck(newNode)}
colChkNode[colChkNode.length]=newNode
newNode.checkBox=chkBox
return newNode
}
alai_tree.prototype.oncheck=new Function("return true;")
}
alai_tree_check()