How to change the column order in a multi-column index using fluent-nhibernate?

Question

How to change the column order in a multi-column index?
I.e:

mapping.References(x => x.SomeReference).SetAttribute("index", "IX_index");
mapping.Map(x => x.SomeField).SetAttribute("index", "IX_index");

Produces the following Schema:

create index IX_index on ApplicantProgramDatas (SomeField, SomeReferenceId)

But I want to get:

create index IX_index on ApplicantProgramDatas (SomeReferenceId, SomeField)