Honestly I'm not sure. Personally I would prefer it to be just:
```
.border-radius(@radius: 5px) {
    .border-radius: @radius;
}
```
or
```
.color(@color: #ff0) {
    color: @color;
}
```
(Hoping that this won't look too simple so that one wonders: "Eh? Why I would need that mixin thing at all?"). But I'm not sure. Let's wait for other opinions?