At the end of page 45, there is a code snippet as follows:

path_with_length(a, f, [Path, Length]).

but this code brings Prolog into an error like below:

ERROR: Unknown procedure: path_with_length/3
ERROR:     However, there are definitions for:
ERROR:         path_with_length/4
false.

Probably we need to remove needless square brackets like below:

path_with_length(a, f, Path, Length).

Thanks for checking this post!