johannes kuester writes:
... to include the upright "d" seems
arbitrary, as other upright glyphs aren't taken into account.
this is not arbitrary, and is there for the same reason that an
upright partial sign is included among the "extra greek-like
material" -- it is to represent the differential operator, which
is upright according to an iso standard for math notation (whose
reference number i don't remember at the moment). since that
standard was developed by engineers, not mathematicians, actual
practice in those two communities may differ, but the fact remains
that the upright "d" is standardized and the (more familiar to me)
italic "d" is not.