An Excellent Driver (day 1) [SLiSW]

/*First time with prolog. It looks good*/
%first exercise
%----------lib.pl-----------------
books(book1, writerA).
books(book1, writerB).
books(book2, writerB).
books(book3, writerC).
books(book4, writerA).
books(book5, writerD).
books(book6, writerE).
books(book7, writerB).
books(book8, writerE).
books(book9, writerC).
books(book10, writerF).
books(book11, writerA).
books(book12, writerB).
books(book13, writerB).
books(book14, writerC).
books(book15, writerD).
books(book16, writerE).
%-------------------------------
/*then in command line is like this
?- ['lib.pl'].
% lib.pl compiled 0.00 sec, 16 clauses
true.

?- books(What, writerA).
What = book1 ;
What = book4 ;
What = book11.
*/

%exercise 2
%-------------------music.pl----------------
singer(singerA, rock).
singer(singerB, metal).
singer(singerC, rb).
singer(singerD, jazz).
singer(singerE, latin).

instrument(singerA, guitar).
instrument(singerA, piano).
instrument(singerB, guitar).
instrument(singerC, piano).
instrument(singerD, saxophone).
instrument(singerC, guitar).
%---------------------------------------
/*command line
?- ['music.pl'].
% music.pl compiled 0.00 sec, 7 clauses
true.

?- instrument(Who, guitar).
Who = singerA ;
Who = singerB ;
Who = singerC.
*/