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
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