static L<Class> nonAbstractClassesInRealmImplementing(Class realm, Class base, L<S> candidateClassNames) {
  new L<Class> l;
  for (S name : candidateClassNames) pcall {
    Class c = _getClass(realm, "main$" + name);
    if (c != null && !isAbstract(c) && isSubtypeOf(c, base))
      l.add(c);
  }
  ret l;
}