Yes, it allows arbitrary keys. The only one that's reserved by the database is `$reql_type$`, and it's used for serialization of data structures that don't have a native JSON representation (time, binary objects, geometry objects, etc.)
So if I try to define `$reql_type$`, it will throw an error? That means that I can't store arbitrary data / use user data as a key. Can't something be done about that at the driver / protocol level (eg. translating literal '$' to '$$' and back again)?
