Is remove safe using foreach

[lili via Digitalmars-d-learn]

```
int[string] aa = ["ok":1, "aaa":2, "ccc":3, "ddd":4];
        foreach (k ; aa.byKey)
        {
                if (k == "aaa") {
                aa.remove(k);
                aa["ww"] = 33;
                }

                if (k == "ww") {
                aa.remove(k);
                aa["vv"] = 33;
                }
        }

writeln(aa); // output ["ok":1, "ddd":4, "vv":33, "ccc":3] is ok
```