Help to find an error

[zet]

Hello.
I'm trying to create xxx(-term) predicate.

I've write such a code:

Bool xxx(PlTerm z)
{
  PlTerm ter = Mk_Integer(1234);
  z = ter;
  return TRUE;
}

After succesfull compile, I've started new executable and enter:
xxx(Z).

Result was:
Z = 0

Can somebody tell me Whats wrong?
Thanks in advance.