I found AA initialization have a strange effect:
```d
string[string] map; // same as `string[string] map =
string[string].init;
writeln(map); // output: []
string[string] refer = map; // make a reference
refer["1"] = "2"; // update the reference
writeln(map); // output: []. The reference does not affect the
original.
```
But if I do an assignment first, the reference works:
```d
string[string] map; // same as `string[string] map =
string[string].init;
writeln(map); // output: []
map["1"] = "0";
string[string] refer = map; // make a reference
refer["1"] = "2"; // update the reference
writeln(map); // output: ["1":"2"]. The reference does affect.
```
So if I want an empty AA and want to use a variable to refer to
it later, I have to do this:
```d
string[string] map; // same as `string[string] map =
string[string].init;
map["1"] = "0";
map.remove("1"); // assign and then REMOVE!
writeln(map); // output: []
string[string] refer = map; // make a reference
refer["1"] = "2"; // update the reference
writeln(map); // output: ["1":"2"]. The reference does affect.
```
which looks awkward.
Is it because `string[string].init == null` ? If so, how do I
specify an empty AA which is not null? Neither `[]` or `[:]`
seems to work.
