代码如下，想问下最后一个语句怎么理解？
```
type Domain_list_t map[string]interface{}
var DomainList Domain_list_t
//DomainList.txt 文件存放域名列表，一行一个
func LoadOrCreateDomainList() bool {
buf, err := ioutil.ReadFile("./Domainlist.txt")
if err != nil {
return false
}
DomainList = make(Domain_list_t)
//domain为Domainlist.txt里的一行，假设是第一行是 www.baidu.com
for _, domain := range strings.Split(string(buf), "\n") {
//subs为domain的各级域名组成的切片 {www,baidu,com}
subs := strings.Split(strings.Trim(domain, "\r "), ".")
if len(subs) == 0 || len(domain) == 0 || domain[0] == '#' {
continue
}
top := DomainList
//i=2时
for i := len(subs) - 1; i >= 0; i-- {
//sub[2]="com"，top["com"]==nil
if top[subs[i]] == nil {
//初始化top["com"]
top[subs[i]] = make(Domain_list_t)
}
if i == 0 {
top[subs[0]].(Domain_list_t)["_"] = true
}
//这里不明白，为啥 top["com"] 后面还能加点号，为啥 Domain_list_t 要加括号
top = top[subs[i]].(Domain_list_t)
}
}
```
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