Fixing up the mod function to produce only positive results?



I've been doing some modular arithmetic with Maxima, and I don't like
the default behaviour than mod(m,n) returns a value between -(n-1)/2
and n/2 (roughly speaking).  For example.

mod(18,7);

returns -3.

But I want my values to be positive, in the range 0..n-1.  How can I
do this?  Is there a local variable I can set, or do I have to edit a
lisp file (which I'd rather not do)?

cheers,
Alasdair