alternate attribute types in xsd

Hi,

I am reaching out to this list, as I did not succeed in finding the
answer in the archives, the web or even in the specs, hoping that
someone here, possibly having contributed to the specs, may know
better.

Please forgive me if either I have not found what I should have or if
this question would have been better asked elsewhere.l

Essencially, the question is something like " how can conditional
(e.g. alternative) attribute types be defined in xsd.?

If the answer is that "it just can't", or that "the feature is not
supported", I would appreciate understanding "why?", especially as it
seems that attribute nodes are nodes, that nodes can have types and
that attributes types are types.

There seems to be important use cases that would require the feature,
as well as straight orthogonal design.

Thank you,

Akhu