sclass WithProbability<A> extends Var<A> {
  double probability; // assumed between 0 and 1
  
  *() {}
  *(double *probability, A a) { super(a); }
  
  toString {
    ret "p=" + formatDouble_noLeadingZero(probability, 2) + ": " + str(get());
  }
  
  double probability() { ret probability; }
}