FD: Arithmetic Constraints

I was trying to express a constraint like C #= abs(A) in fd. After a few
attempts it seemed that none of the usual arithmetic constraints, appart
from the basic operations (+, -, * and /) can be easily represented.

Is this true? Or am I just being a clueless newbie?
Is the way around to write those constraints from scratch as user defined
ones?

--Rui Mendes