code: private void appendToTrash(HashSet<Key> t, Vec[] vec) {
      for (Vec v : vec) t.add(v._key);
    }
    /** Delete all vectors in given trash. */
    private void cleanupTrash(HashSet<Key> trash, Futures fs) {
      for (Key k : trash) UKV.remove(k, fs);
    }
  }
docs: Put given vectors into a given trash.