有关map[string]interface{}用法求教

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)
        }
    }