function  [clock,EL,ND]=departure(clock,EL,NA,ND,mu)
%servicet is a generator of a random  service time.
% advance clock, update number of departures, 
clock=EL(2);
   ND=ND+1;
   %schedule a new service  ifsystem non-empty
   if (NA>ND)
      EL(2)=clock+servicet(mu);
   else
      EL(2)=99999;
   end