// Note: this loads all classes declared in program
static <A> L<Class<? extends A>> nonAbstractClassesImplementing(Class<A> base) {
  L<Class<? extends A>> l = new L;
  for (S name : myClasses()) {
    Class c = _getClass(name);
    if (!isAbstract(c) && isSubtypeOf(c, base))
      l.add(c);
  }
  ret l;
}