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