As you know, Nil in Scala is an object of immutable collections. I have a problem with my custom mutable collection, for example 

```
class ResourceMap extends mutable.HashMap[String, Int] {
   ....
    def -(o: ResourceMapType):ResourceMap = {

        if (....) {
             ......
        } else {
            Nil // Exception : raises the `change return type to Nil.type`
        }
    }
}

```

And my question is how can I return Nil in the `-` method?