static <A> void removeAll(Collection<A> a, Collection<A> b) {
  if (a != null && b != null) a.removeAll(b);
}

static <A, B> void removeAll(Map<A, B> a, Collection<A> b) {
  if (a != null && b != null)
    for (A x : b)
      a.remove(x);
}