AA with complex keytype?

[Joel Salomon]

> > Mathematically speaking, a group like ℤ₃ is unordered,
> > precisely for the reason given.  If we need to impose an ordering
> > for array purposes (I don’t understand that bit, just
> > (mis)repeating something I’ve seen said on the subject), then
> > give the ordering lexicographic properties. 
> 
> It is faster to not impose an ordering on an unordered set.

Do you know where D insists that “some” ordering be applied to a type,
and why?