But the dimension of infinite space is also resistant (i.e., causing stress) and relinquished on the way to emptiness. And the dimension of nothingness also is resistant per MN121.
So what MN121 does is show us that all things are resistant and causing stress except Emptiness. And the very odd oddest thing about this is that the dimension of nothingness is associated with equanimity. There is the implication that equanimity itself arises only with respect to a self and that emptiness also relinquishes equanimity.
The term rūpasaṅgaho only occurs in Ds2.2.2, Ds2.2.3 and DN33. Ds2.2.3 was translated by Caroline Rhys-Davids. And Ds2.2.3 ends with:
What is that form which is invisible, nonimpingeing, and included in the sphere of [mental] states?Sex … and bodily nutriment.
The above matches what Mat said about form, but makes absolutely no sense to me. How on earth is sex nonimpingeing?
The answer might be locked in this Pali from Ds2.2.2:
2.2.2.11. Ekādasaka
Ekādasavidhena rūpasaṅgaho—
Cakkhāyatanaṃ, sotāyatanaṃ, ghānāyatanaṃ, jivhāyatanaṃ, kāyāyatanaṃ, rūpāyatanaṃ, saddāyatanaṃ, gandhāyatanaṃ, rasāyatanaṃ, phoṭṭhabbāyatanaṃ, yañca rūpaṃ anidassanaappaṭighaṃ dhammāyatanapariyāpannaṃ.
Evaṃ ekādasavidhena rūpasaṅgaho.
Ekādasakaṃ.
Mātikā.
I do think my “solution” is idiosyncratic and in need of revision, but we would need Bhante @Sujato to help us out on this mysterious verse from DN33. My “solution” was just realizing that Bhante’s translation of invisible and non-resistant could be assigned a relevant personal interpretation as Emptiness.