simplification based on assumptions



Hi I'm new to maxima so forgive me if this is a fairly basic question.
I've got some equations that use nested max and min functions and some
>= constraints between some of the variables. I know that certain
things can be simplified but I'm not sure how to get maxima to do it.

As an example I had the following exchange with maxima 5.9.3:

(%i1) assume(a > b);
(%o1)                               [a > b]
(%i2) max(a, b);
(%o2)                                  a
(%i3) max(a, b - 5);
(%o3)                            max(b - 5, a)

Surely maxima is able to see that 'max(b - 5, a)' is always 'a'?

Thanks