    """
       An [[Array]] instance referring to another instance via one
       of its elements.

       For example, given:

           value arr = Array({"hello"});
           value context = serialization();
           value refs = context.references(arr);
           assert(is Element elementRef = refs.find((element) => element is Element));
           assert(elementRef.referred(arr) == "hello");
           assert(elementRef.index == 0);
    """
    shared sealed interface Element /*<Instance>*/
            satisfies ReachableReference /*<Instance>*/{
        "The index of the element in the Array which makes the reference."
        shared formal Integer index;
    }