--- a/3-Prolog/day1-self-study.pl	Tue Sep 26 20:26:45 2017 +0100
+++ b/3-Prolog/day1-self-study.pl	Tue Sep 26 20:45:26 2017 +0100
@@ -1,26 +1,38 @@
 % Make a KB of books and authors and query it.
 book(the_long_earth).
 book(night_watch).
+book(science_of_discworld).
 book(lord_of_the_rings).
 book(route_666).
 book(krokodil_tears).
 author(terry_pratchett).
 author(stephen_baxter).
+author(jack_cohen).
+author(ian_stewart).
 author(jrr_tolkein).
 author(jack_yeovil).
 wrote(terry_pratchett, the_long_earth).
 wrote(stephen_baxter, the_long_earth).
 wrote(terry_pratchett, night_watch).
+wrote(terry_pratchett, science_of_discworld).
+wrote(jack_cohen, science_of_discworld).
+wrote(ian_stewart, science_of_discworld).
 wrote(jrr_tolkein, lord_of_the_rings).
 wrote(jack_yeovil, route_666).
 wrote(jack_yeovil, krokodil_tears).
 
-collaborated(Author1, Author2) :- \+(Author1 = Author2), author(Author1), author(Author2),
+collaborated(Author1, Author2, Book) :- \+(Author1 = Author2), author(Author1), author(Author2),
     wrote(Author1, Book), wrote(Author2, Book), book(Book).
+
+collaborated(Author1, Author2) :- collaborated(Author1, Author2, Book).
 % collaborated(terry_pratchett, stephen_baxter). returns true, but collaborated(Author1, Author2).  just says "no", as does
-% collaborated(terry_pratchett, OtherAuthor). - for some reason it isn't filling in answers.
+% collaborated(terry_pratchett, OtherAuthor). - for some reason it isn't filling in answers. It does, however, correctly
+% say that Pratchett never collaborated with Pratchett.
 %
 % If I remove "\+(Author1 = Author2)" then it successfully returns stephen_baxter for terry_pratchett but also returns
 % Pratchett himself as a valid answer as well.
 %
-% wrote(terry_pratchett, Book). works to return all books by an author, though.
\ No newline at end of file
+% wrote(terry_pratchett, Book). works to return all books by an author, though.
+%
+% However, having added a three-argument version, it can correctly identify the books that someone collaborated on if
+% you supply both authors.
\ No newline at end of file