// P + Q = R
	// (xp, yp) + (xq, yq) = (xr, yr)
	// Y = (yq - yp)/(xq - xp)		(isn't that some kind of distance)?
	// xr = Y^2 - xp - xq
	// yr = Y(xp - xr) - yp
	// assert: neither P or Q are point at infinity, and Px != Qx
	// if P or Q is point at infinity, R = other point
	// if P = Q, double instead
	// if Px == Qx, set P to point at infinity