Persisting one table per class hierarchy using NHibernate without discriminator?

I have an interface and a class which implements this interface.

public interface IPhase {
    string Description { get; set; }
    int Id { get; }
    string Phase { get; set; }
}

public class Phase : IPhase {
    // Implementation here...
}

Now, using NHibernate 2.1.2.GA, I wish to use a table-per-class-hierarchy to map the interface and its implementor, and indeed, I don't need a discriminator, as this implementor will be the only class persisted in this table. I haven't used NHibernate for about a year and a half now, and I'm suffering some memory blanks here...

I have read this question and answers which is related, except I'm not using FNH.
NHibernate Mapping: Save hierarchy to single table without discriminator

1. I wonder whether the discriminator attribute is obligatory while using subclass?

2. What shall my XML mapping look like in this particular context?

Thanks kindly for your help!

Of course NHibernate needs a discriminator for a table-per-class-hierarchy mapping, how should it identify the different subclasses when getting a row from the database otherwise?

If there is only one implementation of the interface, why do you want to map it? Just map the class as a normal entity (without inheritance).

edit: Forgot that you might have references to the interface in your mapping. In that case you could try a table-per-concrete-class mapping marking the base class (interface) with abstract="true" like described here: http://nhibernate.info/doc/nh/en/index.html#inheritance-tableperconcrete