Evaluating arithmetic expressions with macros (eval)

Dear all,

I find myself stuck with something which I assume is trivial. I define:

m4_define([X], [9])
m4_define([Y], [3])

And I would like to define Z as being the arithmetic sum of X and Y and
can seem to get it.

I tried several variations of eval but had no success. I understand
that all the macros need to be expanded before eval is called but I
don't understand how to do it.

Any help warmly appreciated.

Thanks in advance,

Seb.