> You don't invent them, they exist. I can't agree to dismiss this 
> suggestion without further thought. It might end up being 
> non-sensical, maybe not. It merely suggests a way, to derive new 
> values, whose type happen to be a supertype of the original one. Why 
> not ? It may be considered to the Haskell-like "derive" feature, and 
> such. It is just a way to build new values out of known one.
>
Formally, can you explain what you're looking for here, and how it's not 
congruent with inheritance?

~~ Robert.