docs: private RelationReified reify() {
          if (relationStructure.isReified()) return relationStructure.reify();
  
          //Get the role players to transfer
          Map<Role, Set<Thing>> rolePlayers = structure().allRolePlayers();
  
          //Now Reify
          relationStructure = relationStructure.reify();
  
          //Transfer relations
          rolePlayers.forEach((role, things) -> {
              Thing thing = Iterables.getOnlyElement(things);
              assign(role, thing);
          });
  
          return relationStructure.reify();
      }