* Behaves like an {@link java.util.Optional Optional} that is always empty. + *
+ * Why would anyone want such a thing? + *
+ * The domain of a {@link Listing} plays a role somewhat like a datatype:
+ * it indicates the nature of the values it can reference.
+ * Often, the domain is a {@link Catalog} that explicitly lists all the allowed values,
+ * but sometimes you don't want to list all the possible values in the bosk state;
+ * you just want to be clear about what the allowed values are.
+ * For example, there may be a large or infinite number of possible values;
+ * or there might be nothing to store about the value besides its ID.
+ * In that case, you can use a Phantom<Catalog<T>>,
+ * and reference that as your domain.
*/
public final class Phantom