IFR use of handheld GPS

Dane Spearing wrote:

Section 1-1-19-d of the AIM addresses the general requirements
for conducting any GPS operations under IFR. Section 1-1-19-d1a
explicitly states (and I quote):

"Visual flight rules (VFR) and hand-held GPS systems are not authorized
for IFR navigation, instrument approaches, or as a principal instrument
flight reference."

Okay. Good find. Now, some will always bring up the fact that the AIM
is not regulatory, but it is an official FAA publication and therefore
cannot be disregarded. I somehow missed that sentence over the years.
So I suppose weaseling around it by claiming its really GPS-assisted
dead reckoning is necessary.

So if the AIM says that handhelds are not authorized for IFR
navigation, there must be a rule somewhere, right?