return Collections.unmodifiableList(new ArrayList<>(versions));
    }

    public void setReaders(Collection<WorkspaceReader> readers) {
        requireNonNull(readers, "readers");
        // skip possible null entries
        this.readers = Collections.unmodifiableList(
                new ArrayList<>(readers.stream().filter(Objects::nonNull).collect(Collectors.toList())));
        Key key = new Key(this.readers);