ofile := "/tmp/simul.dat" ;;
gnuplot := "/usr/bin/gnuplot" ;;

type chemical = `Prey | `Predator ;;

type population = [chemical]bag ;;

S0 := 1500 * `Prey + 50 * `Predator ;;

trans rules = {

  `Prey ={ C = 1. }=> `Prey, `Prey;

  `Predator ={ C = 1. }=> <undef>;

  `Predator, `Prey ={ C = 0.001 }=> `Predator;

  `Predator, `Prey ={ C = 0.001 }=> `Predator, `Predator;

} ;;

rules[strategy = `gillespie](S0) ;;

fun output(S) = (
  if iteration % 10 == 0
  then
    let nb_preys = count(`Prey,S)
    and nb_predators = count(`Predator,S)
    in
      ofile << tau << " " << nb_preys << " " << nb_predators << "\n"
  fi;
  S
) ;;

rules[strategy = `gillespie,
      prelude = output,
      interlude = output,
      postlude = output,
      iter = 100000
     ](S0) ;;

gnuplot << "  plot \"" + ofile + "\" u 1:2 w l\n" ;;
gnuplot << "replot \"" + ofile + "\" u 1:3 w l\n" ;;