[Aldor-l] Re: [Axiom-developer] operations working in general, but not in special cases -- help needed

[Christian Aistleitner]

Hi Bertfried,

> Category Matroid
>
>   expoerts dual
>
> then
>
> Category GraphMatroid
>
>   inherits Matroid
> =>modifies dual -> dual= if has planar then dual else error
>   exports  new functionality

thereby, you would demolish the semantics of the dual of Matroid.

I would strongly discourage from using such a solution. (Compare  
covariance and contravariance for parameters)
If I understood you correctly, right now such a solution is not possible  
in Axiom anyways. And this is actually good!

According to Martin's description of matroids and "graphical matroids", a  
"graphical matroid" is _not_ a matroid.
So a "graphical matroid" should _not_ be modelled as a specialisation of  
matroid.

--
King regards,
Christian