*/
    ValueProducer getProducer();

    boolean calculatePresence(ValueConsumer consumer);

    enum ValueConsumer {
        DisallowUnsafeRead, IgnoreUnsafeRead
    }

    /**
     * Carries information about the producer of a value.
     */
    interface ValueProducer {
        NoProducer NO_PRODUCER = new NoProducer();

        def a = property()
        def b = property()
        def c = elementProperty()
        def property = property()
        property.disallowUnsafeRead()

        property.addAll(a)

        a.addAll(b)
        a.attachOwner(owner(), displayName("<a>"))

        b.attachOwner(owner(), displayName("<b>"))

        property.add(c)

        def a = property()
        def b = property()
        def c = valueProperty()
        def property = property()
        property.disallowUnsafeRead()

        property.putAll(a)

        a.putAll(b)
        a.attachOwner(owner(), displayName("<a>"))

        b.attachOwner(owner(), displayName("<b>"))

        property.put("c", c)