static <A> bool syncRemove(Collection<A> c, A b) {
  if (c == null) false;
  synchronized(collectionMutex(c)) { ret c.remove(b); }
}

static <A> A syncRemove(L<A> l, int idx) {
  if (l == null) null;
  synchronized(collectionMutex(l)) { ret l.remove(idx); }
}

static <A, B> B syncRemove(Map<A, B> map, A key) {
  ret map == null ? null : map.remove(key);
}