There is a Set field 'javax.imageio.spi.PartiallyOrderedSet#nodes' which is initialized as keySet of Map field `poNodes`

    // Maps Objects to DigraphNodes that contain them
    private Map<E, DigraphNode<E>> poNodes = new HashMap<>();

    // The set of Objects
    private Set<E> nodes = poNodes.keySet();

The field 'nodes' is redundant: we can always use methods of 'poNodes' directly, without additional delegation.