I had the same thought skimming the PDF, but he does address it. One of his suggestions is to use an error value as the point at infinity. So I suppose the poin

I had the same thought skimming the PDF, but he does address it. One of his suggestions is to use an error value as the point at infinity. So I suppose the point is the coding looks the same, but you're taking a more principled view of what you're doing for reasoning purposes.