code: public ImmutableSortedSet<Name> snapshot() {
      ImmutableSortedSet.Builder<Name> builder =
          new ImmutableSortedSet.Builder<>(Name.displayOrdering());
  
      for (DirectoryEntry entry : this) {
        if (!isReserved(entry.name())) {
          builder.add(entry.name());
        }
      }
  
      return builder.build();
    }
docs: Creates an immutable sorted snapshot of the names this directory contains, excluding "." and
  "..".