static Set<Class> allInterfacesImplementedBy(Class c) {
  if (c == null) null;
  new HashSet<Class> set;
  allInterfacesImplementedBy_find(c, set);
  ret set;
}

svoid allInterfacesImplementedBy_find(Class c, Set<Class> set) {
  if (c.isInterface() && !set.add(c)) ret;
  for (Class intf : c.getInterfaces())
    allInterfacesImplementedBy_find(intf, set);
  c = c.getSuperclass();
  if (c != null) allInterfacesImplementedBy_find(c);
}