Disagree, they are two separate things.
Posted under General
Couldn't you just include and exclude flat_chest to differentiate the two, and not need a naizuri tag at all? Well, unless you want to be covered in the event of a flat chested person happening to be in the same picture as someone else performing paizuri...
I don't think this is a good idea at all.
The whole point of naizuri is that paizuri is impossible without sufficient cleavage to squeeze a penis between. The two are somewhat related, but mutually exclusive.
If you want to be that technical, naizuri is more like a variant of frottage, since it involves rubbing the penis against a surface rather than any kind of enclosure or insertion of it.
*still is unable to see why a derivation doesn't logically call for association*
*well, OK, is able, but is naizuri weren't introduced, paizuri would still be in place, and even the flat chests involved can provide some enclosure, even if there's not a full wrap-around (as some regular paizuri pics undoubtedly exhibit)*
Updated