My only quibbles are with 5A, which led me astray for a while. (A) My understanding is that ABACI were invented independently in several civilizations, but definitely Sumerian/Babylonian well before Greece; and (B) I don't think they were handheld, especially in early times (some of the older ones were "sandboard" structures and had to be on a stable surface).
Well, regardless of where they were first invented, they were used in ancient Greece, so I guess that part is okay. "Greek" works a lot better with the verse's meter than "Sumerian". I assume you're right, though, about the handheld bit -- sorry! I do like the image of an ancient Greek nerd walking around with a handheld abacus. I would have been one myself!
For this critical part of a body's key tract, / A suitable symbol: two little dots, stacked.
1
2
3
4
5
6
Orizzontali
1A
For this critical part of a body's key tract, / A suitable symbol: two little dots, stacked.
4A
To prove something false, a debater might say. To invalidate or to negate. / Look at it wrong, though, and you get something else: A potato! (And isn't that great?)
5A
It's one of those irregular pluralized words -- / They're handheld devices for ancient Greek nerds.
6A
Left on the left, right on the right, with agreement found in between. / A stratum in the sediment? This is what is seen.
Verticali
1D
To curb, to limit, to reduce, and perhaps to restrict all around, / Or something at one end of a mean and mangy hound.
2D
A place filled with items flat and rectangular / Displayed in order for easiest view. / Its items are there for you to take with you. / At some point, though, they're gonna be due!
3D
My pecan pie has more pecans than theirs. / It makes me wackier, odder "upstairs".