"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;
}