Doppio scalar field & arithmetic

[cathieyun]

Unsure if this is necessary (maybe it's enough to just do point operations over the doppio points, without a doppio scalar field?). Also unsure what this scalar field would look like.

[hdevalence]

Yes, I think we'll need to implement the scalar field. I'm not sure what the modulus is, we should probably get a hold of the sage script @ebfull used and pull it into this repo so that there's a way to work with the curve in sage.