如何使用fluent-nhibernate更改多列索引中的列顺序？

public class PersonMap : ClassMap<Person>
{
    public PersonMap()
    {
        Table("Persons");
        Id(x => x.Id);
        Map(x => x.FirstName);
        Map(x => x.LastName);
        Index(x => x.FirstName, x => x.LastName);
    }
}

public class PersonMap : ClassMap<Person>
{
    public PersonMap()
    {
        Table("Persons");
        Id(x => x.Id);
        Map(x => x.FirstName);
        Map(x => x.LastName);
        Index(x => x.LastName, x => x.FirstName);
    }
}

public class CreatePersonsTable : Migration
{
    public override void Up()
    {
        Create.Table("Persons")
            .WithColumn("Id").AsInt32().PrimaryKey().Identity()
            .WithColumn("FirstName").AsString()
            .WithColumn("LastName").AsString();

        Create.Index("IX_Persons_FirstName_LastName")
            .OnTable("Persons")
            .OnColumn("FirstName")
            .OnColumn("LastName");
    }

    public override void Down()
    {
        Delete.Table("Persons");
    }
}