Specifying the "d:" field only worked for
NumberValueRangeProcessor in older versions of Xapian, such
as the one in Debian wheezy (libsearch-xapian-perl=1.2.10.0-1)
This slipped through since I rarely use wheezy, anymore, and
perhaps nobody else does, either. Perhaps wheezy support may be
dropped, soon.
Unfortunately, this requires a schema version bump.