dot中subgraph显示没有区域边界（边框）

下面是一段dot语言的代码：

digraph G {
subgraph main {
    label="main";
    bb0000a7c4;
    call_reset;
    bb0000a7c4 -> call_reset;
    call_reset [shape=polygon];
    call_reset -> bb0000a7d4;
    bb0000a7d4 -> bb0000a830;
    bb0000a7f0 -> call_my_cos;
    call_my_cos [shape=polygon];
    call_my_cos -> bb0000a80c;
    bb0000a80c -> bb0000a830;
    bb0000a830 -> bb0000a83c;
    bb0000a830 -> bb0000a7f0;
    bb0000a83c -> bb0000a89c;
    bb0000a848 -> call_encode;
    call_encode [shape=polygon];
    call_encode -> bb0000a884;
    bb0000a884 -> bb0000a89c;
    bb0000a89c -> bb0000a8a8;
    bb0000a89c -> bb0000a848;
    bb0000a8a8 -> bb0000a910;
    bb0000a8b4 -> call_decode;
    call_decode [shape=polygon];
    call_decode -> bb0000a8d8;
    bb0000a8d8 -> bb0000a910;
    bb0000a910 -> bb0000a91c;
    bb0000a910 -> bb0000a8b4;
}

}
然后运行dot命令后，生成的图不带边框。与样例代码比较过，没有看出明显区别。

main前面加上cluster就可以

subgraph的名字必须有cluster前缀才生效