The Sorgenfrey line is not consonant

In Exercise 5.4.12 of the book, I ask the reader to prove that neither the space of rationals, Q, nor the Sorgenfrey line, R_ℓ, is consonant. But the proofs I had in mind were much too simple-minded to stand any chance of succeeding, hence I classified this as important blooper #5 in the list of errata. Good news: Showing that R_ℓ is not consonant is not that hard, finally. I will explain the argument in the full post. This will also be an excuse to explain some additional topological properties of R_ℓ, an introduction to hereditary Lindelöfness (we will see that R_ℓ is hereditarily Lindelöf, although it is not second-countable), and a few additional things in the appendices.