Exclusive + Required

I'd love to require exactly one of a few choices, e.g.

``` python
s = Schema({
Exclusive('project_id', 'project', require=True): Coerce(int),
Exclusive('project_name', 'project', require=True): unicode
})
```

(With `require=True` not existing now, but if something like it did, that would be great.)

It's likely this is possible, but I'm not sure how to do it yet. (Is it?)

I'd like `s({})` to fail, for examine, but a schema containing either one of the keys (as is now the case with Exclusive) to pass.