In reply to Bhaskar44:
You probably want
constraint r_delta {at > const'(at);}
More likely, you want
constraint r_delta {at inside {[const'(at)+min_delta: const'(at)+max_delta]};}
where the deltas are to keep the arrival times within some range.
Also use
rand instead of
randcfor very large random numbers. randc may be very costly.