sclass RandomAndVary_Record<A> implements RandomAndVary<A> {

  F0<A> makeInstance;

  new Map<S, RandomAndVary> handlers; // key = field name

  *(Class<A> theClass, O... params) {

    makeInstance = func -> A { nu(theClass) };

    addAll(handlers, paramsToMap(params));

  }

  *(A prototype, O... params) {

    makeInstance = func -> A { shallowClone(prototype) };

    addAll(handlers, paramsToMap(params));

  }

  public A randomInstance() {

    A a = makeInstance!;

    for (S field, RandomAndVary handler : handlers) {

      setOpt(a, field, handler.randomInstance());

    }

    ret a;

  }

  public A varyInstance(A a) {

    a = shallowClone(a);

    S field = random(keys(handlers));

    RandomAndVary handler = handlers.get(field);

    O o = getOpt(a, field);

    if (o == null) null;

    setOpt(a, field, handler.varyInstance(o));

    ret a;

  }

}