"The reader is assumed to be familiar with J and C." And anyone reading this at the time would have been familiar with APL as well. It's not intended to be be

"The reader is assumed to be familiar with J and C." And anyone reading this at the time would have been familiar with APL as well. It's not intended to be beginner friendly. Like J, and like the original J dictionary, the values here are brevity, compactness, and essence. There is plenty of other more beginner friendly material on J out there.