Plus one to this. $< is never defined before being used, as far as I could find.

It is defined in the exercise itself:

...$<, which means 'the first dependency of the current rule'.

But this can be made clearer. Something like, "For this exercise you will also need another automatic variable, $<, which means 'the first dependency of the current rule' ?

I was trying to avoid introducing it by an example explicitly as some attendees complained that some of the exercises were too much of the copy-paste-customise approach.