How to setup Fluent NHibernate Many-To-Many automapping using Set instead of Bag?

Question

http://www.codinginstinct.com/2010/03/nhibernate-tip-use-set-for-many-to-many.html

I want to do the way the author suggested for fluent nhibernate many-to-many but use automapping instead of HBM file.

Here are my two entities

 public class User
    {
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
        public virtual Iesi.Collections.Generic.Set<City> Cities { get; set; }

}

 public class City{
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
        public virtual Iesi.Collections.Generic.Set<User> Users { get; set; }

}

Tried with HashSet, IList, and Set. But when I looked at the HBM files generated by calling automapping output method:

 var autoMappings = new AutoPersistenceModel().AddEntityAssembly(entityAssembly).Where(x => x.Namespace.EndsWith("Domain"));

autoMappings.WriteMappingsTo((@"C:\TEMP");

It's still bag type

<bag inverse="true" name="Users" table="MNUserCity" mutable="true">
      <key>
        <column name="CityId" />
      </key>
      <many-to-many class="MyApp.Entity.Domain.User, MyApp.Entity, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null">
        <column name="UserId" />
      </many-to-many>
    </bag>

Is there any conventions/override I can use in Fluent NHibernate to alter the collection type for all the ManyToMany in the app domain? I looked at IHasManyToMany convention but no clue.

Anyone can help? Thanks.

BTW, I'm using latest build in http://github.com/jagregory/fluent-nhibernate