In Proposition 1.5.8, there are a few errors. First, in the statement
it should say "Every nonstraight angle has a unique bisector"
(thanks to Nick for noticing this). Second, in the second
sentence of the proof, it should say angle BAD' ~angle C'AD' (i.e., D' instead of D). In addition to the above, there is one
more error. Find it, and then fix the proof. (Challenge: there are two
ways to fix the proof; can you find both?)