    "Exposes the instances directly reachable from a given instance."
    shared sealed interface References/*<Instance>*/
        // could be generic
            satisfies {<ReachableReference/*<Instance>*/->Anything>*} {

        "The instance"
        shared formal Anything/*<Instance>*/ instance;

        "The references that are reachable from the [[instance]]."
        shared formal Iterable<ReachableReference/*<Instance>*/> references;
    }