why is it a class property cannot be used like a.b ~= c; ?

[someone]

```d
public class cSomething {

    private:

    dstring pstrWhatever = null;

    public:

    @safe dstring whatever() { return pstrWhatever; }
    @safe void whatever(const dstring lstrWhatever) { pstrWhatever 
= lstrWhatever; }

}

void main() {

    cSomething lobjSomething = new cSomething();
    lobjSomething.whatever = r"abc"d;
    lobjSomething.whatever ~= r"def"d; /// Error: 
`lobjSomething.whatever()` is not an lvalue and cannot be modified

}
```