sclass RandomAndVary_Record<A> implements RandomAndVary<A> {
  Class<A> theClass;
  new Map<S, RandomAndVary> handlers; // key = field name

  *(Class<A> *theClass, O... params) {
    addAll(handlers, paramsToMap(params));
  }
  
  A randomInstance() {
    A a = nu(theClass);
    for (S field, RandomAndVary handler : handlers) {
      setOpt(a, field, handler.randomInstance());
    }
    ret a;
  }
  
  A varyInstance(A a) {
    a = shallowClone(a);
    for (S field, RandomAndVary handler : handlers) {
      O o = getOpt(a, field);
      if (o == null) continue;
      setOpt(a, field, handler.varyInstance(o));
    }
    ret a;
  }
}