Prolog section update

free-programming-books.md

@@ -761,11 +761,10 @@ See also [TeX](#tex)
 * [Introduction to Prolog for Mathematicians](http://www.j-paine.org/prolog/mathnotes/files/pms/pms.html)
 * [Learn Prolog Now!](http://www.learnprolognow.org/)
 * [Logic, Programming and Prolog (2ed)](http://www.ida.liu.se/~ulfni/lpp/)
-* [Natural Language Processing in Prolog](http://www.informatics.susx.ac.uk/research/groups/nlp/gazdar/nlp-in-prolog/index.html)
 * [Natural Language Processing Techniques in Prolog](http://cs.union.edu/~striegnk/courses/nlp-with-prolog/html/)
 * [Prolog Techniques](http://bookboon.com/int/student/it/prolog-techniques-applications-of-prolog)
 * [Simply Logical](http://www.cs.bris.ac.uk/~flach/SimplyLogical.html)
-* [Visual Prolog 7.2 for Tyros](http://download.pdc.dk/vip/72/books/tyros/tyros72.pdf)
+* [Visual Prolog 7.2 for Tyros](http://download.pdc.dk/vip/72/books/tyros/tyros72.pdf) (PDF)
 
 
 ###PostgreSQL