# HG changeset patch
# User IBBoard <dev@ibboard.co.uk>
# Date 1506711650 -3600
# Node ID 49cb2f7af872d1cd6dd152275c75d6b113af71a4
# Parent  643eec1f9e3a4e09a1b46cdfbf6275c7c8b84688
Fix the Day 1 collaboration problems

diff -r 643eec1f9e3a -r 49cb2f7af872 3-Prolog/day1-self-study.pl
--- a/3-Prolog/day1-self-study.pl	Wed Sep 27 20:48:28 2017 +0100
+++ b/3-Prolog/day1-self-study.pl	Fri Sep 29 20:00:50 2017 +0100
@@ -21,8 +21,8 @@
 wrote(jack_yeovil, route_666).
 wrote(jack_yeovil, krokodil_tears).
 
-collaborated(Author1, Author2, Book) :- \+(Author1 = Author2), author(Author1), author(Author2),
-    wrote(Author1, Book), wrote(Author2, Book), book(Book).
+collaborated(Author1, Author2, Book) :- author(Author1), author(Author2),
+    wrote(Author1, Book), wrote(Author2, Book), book(Book), Author1 \== Author2.
 
 collaborated(Author1, Author2) :- collaborated(Author1, Author2, Book).
 % collaborated(terry_pratchett, stephen_baxter). returns true, but collaborated(Author1, Author2).  just says "no", as does
@@ -35,4 +35,10 @@
 % 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 but it won't supply an author when given one author and the book title.
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+% you supply both authors but it won't supply an author when given one author and the book title.
+%
+% On tracing this and the W&G example then it seems that "\+(…)" doesn't play nicely with variables. The W&G examples
+% worked because we only asked "are these two people friends" questions, not "who is friends with" questions.
+%
+% What we actually need is "\==" (not equal/equivalent). Also, we need to put it at the end so that Author1 and Author2 have
+% been assigned by that point.
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