OLAP, Aggregates, and order of operations

I need to do some OLAP stuff, and I asked previously if there were a way
to pass multiple parameters to an aggrigate function. i.e.:

select mycube(value1, value2, value3) from table group by value1;

I looked through the code and it is non-trivial to do, one would have to
alter the grammar to include a number of parameters, I guess something
like this:

create aggregate (sfunc = myfunct, sfuncnargs=3, stype = int4, basetype1
= int4, basetype2 = int4, ....);

Then change the catalog, and the execution, arrg!

(God I wish I could spend the time I want on PostgreSQL! )

Anyway, short of that....

If I do this:

select mycube(value1) as d1, dimention(value2) as d2, dimention(value3)
as d3 group by value1;

Can I safely assume the following:

(1) mycube() will be called first
(2) Assuming dimention() has no final func, that final func of mycube()
will be called last.