Hi Staffan,
Happy to spot things like this. Your book is excellent; I’m really enjoying it.
I had another one, more a refinement than an error. I’ve been a programmer for 50+ years, so I was around when ASCII was invented. Your description of the “pillow” character on page 34 (of the PDF) is not precise. ASCII is generally considered to be 0-127. The “upper” ASCII plane (where the pillow is at 164) is considered “Extended ASCII” as it requires an 8th bit. There were many names for this extended plane over the years, but to call it unqualified ASCII is imprecise and confused me at first. I’m sensitive to confusion, because if it confuses me, it’ll confuse others. A better way to describe it would be as Extended ASCII, or even better, as Unicode, where it was introduced in 1993.
But honestly, as writers and editors like to say, “kill your darlings.” You could throw this side note away; it has only minor historical curiosity as the currency idea experienced only sporadic adoption, but more importantly, the side note adds nothing to the discussion of the pillow designating the empty set.
Jim