// syncs on a
static <A> bool syncRemoveAllExcept(Set<A> a, Collection<A> b) {
  if (a == null) false;
  synchronized(collectionMutex(a)) {
    ret removeAllBut(a, b);
  }
}

static <A, B> bool syncRemoveAllExcept(Map<A, B> a, Collection<A> b) {
  if (a == null) false;
  synchronized(collectionMutex(a)) {
    ret removeAllBut(a, b);
  }
}

Began life as a copy of #1030249