docs: @SuppressWarnings("deprecation") // The String ctor is deprecated, but fast.
    public Set<String> keys() {
      if (isEmpty()) {
        return Collections.emptySet();
      }
      Set<String> ks = new HashSet<>(size);
      for (int i = 0; i < size; i++) {
        ks.add(new String(name(i), 0 /* hibyte */));
      }
      // immutable in case we decide to change the implementation later.
      return Collections.unmodifiableSet(ks);
    }