Machine readable parts of various Prolog books

lang/prolog/bookcode/

   aitp/      AITP: Code from Shoham's book, "AI Techniques in 
              Prolog".
   aop/       AOP: Code from Sterling's book "The Art of 
              Prolog"
   ppid/      PPID: Code from Covington's book, "Prolog 
              Programming in Depth".
   tutor/     TUTOR: Jocelyn Paine's Logic Programming Tutor