# queue.nas
# valkmjolnir 2021/3/31
var new = func() {
    var (begin, end) = (nil, nil);
    return{
        push: func(elem) {
            var new_node = {
                elem:elem,
                next:nil
            };
            if (begin == nil)
                begin = end = new_node;
            else {
                end.next = new_node;
                end = new_node;
            }
        },
        pop: func() {
            if (begin != nil)
                begin = begin.next;
            if (begin == nil)
                end = nil;
        },
        front: func() {
            if (begin != nil)
                return begin.elem;
        },
        clear: func() {
            begin = end = nil;
        },
        empty: func() {
            return begin == nil;
        }
    };
}