p. 32
This grammar principle is the first to use a double arrow (=>) as
implication symbol, earlier in the text it is a simple arrow (->,
e.g. Head-Complement Schema on p. 21). Later in the text
there's also varying notation for implication (Schema 3 on page 35 is
back to the single arrow). It would be advisable to decide on a single
notation and stick with it.
To do: explain the arrows somewhere.