How to do soft constraints (not FD)?

[anthonym]

predicate(....):-
  predicate_x(....),
  predicate_y(...),
  predicate_z(....).

Suppose I have a predicate and there are some conditions that it must
satisfy (ie the predicate x,y,z). How can I make one of the predicates
into a soft constraint, for example such that the predicate_y does not
have to be true all the times but should maximize such that it is true as
much as possible?

In FD, it is possible to use reified constraints and the fd_maximize()
function, I wonder if I can do something similiar in my case?

Thanks guys. :)

Anthony