public Iterable<JvmNodeReferenceID> allDirectSupertypes(JvmNodeReferenceID classId) {
    // return only those direct supertypes that exist in the graph as Nodes
    return unique(flat(map(getNodes(classId, JvmClass.class), cl -> flat(map(cl.getSuperTypes(), st -> map(getNodes(new JvmNodeReferenceID(st), JvmClass.class), JvmClass::getReferenceID))))));
  }
public Iterable<JvmNodeReferenceID> allDirectSupertypes(JvmNodeReferenceID classId)