I think I have such a script.
Did you finally get yours to work?
Yes I have it on my laptop, if you need it let me know, I have evolved so much since I posted this topic.
if I want to subtract points t=j-d
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def subpoint (xj, yj, xd, yd):
xt = (((yj*yj+2*yd*yj+yd*yd)*modinv (xd*xd-2*xd*xj+xj*xj,N2))-xd-xj)%N2
yt = ((((yj+yd)*(xt-xd))*modinv(xd-xj,N2))+yd)%N2
return (xt, yt)
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modinv modulo division
N2 = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F
But not this much evolved. Lol.