public double chose_rule(Random rd, out rule r){
double a0;
double[] a = get_propensities(out a0);
if (a0 < EPSILON) {
r = new b_rule();
return 0.0;
}
double tau = -Math.Log (rd.NextDouble ()) / a0;
int n = -1, nr = rules.Count;
double s = 0, rx = rd.NextDouble ();
while (n < nr-1 && s < rx) {
n++;
s += a [n] / a0;
}
r = rules[n];
return tau;
}
public void add_rule(rule o){
if (!rules.Contains (o))
rules.Add (o);
}
