This is kind of funny. This works because reduce is defined as follows:

template reduce(alias fun)
{
   alias Reduce!(fun).reduce reduce;
}

I did that to work around some old compiler bugs that have been since fixed. I'd initially intended to define reduce like this:

Unqual!E reduce(alias fun, E, R)(E seed, R r) { ... }

but that wouldn't have allowed Nick's alias, which is quite useful. So it looks like we have a cool idiom.