Understanding Finite-State Machines in Prolog??

[JC0133]

I am taking a programming class using prolog and it is really confusing.

I would like to know if anybody can explain the syntax and logic behind the template code below. Also if possible to provide me with a good prolog and LISP book, and/or some good tutorial websites for either of them?

% basic finite state machine engine

go :-
intro,
start_state(X),
show_state(X),
get_choice(X,Y),
go_to_next_state(X,Y).

intro :-
display_intro,
clear_stored_answers,
initialize.

go_to_next_state(_,q) :-
goodbye,!.

go_to_next_state(S1,Cin) :-
next_state(S1,Cin,S2),
show_transition(S1,Cin),
show_state(S2),
stored_answer(moves,K),
OneMoreMove is K + 1,
retract(stored_answer(moves,_)),
asserta(stored_answer(moves,OneMoreMove)),
get_choice(S2,Cnew),
go_to_next_state(S2,Cnew).

go_to_next_state(S1,Cin) :-
\+ next_state(S1,Cin,_),
show_transition(S1,fail),
get_choice(S1,Cnew),
go_to_next_state(S1,Cnew).

get_choice(_,C) :-
write('Next entry (letter followed by a period)? '),
read(C).

% case knowledge base - user responses

:- dynamic(stored_answer/2).

% procedure to get rid of previous responses
% without abolishing the dynamic declaration

clear_stored_answers :- retract(stored_answer(_,_)),fail.
clear_stored_answers.

 

[JC0133]

I think I have figured out some of the code/syntax. I still don't understand how this part of the code works?

stored_answer(moves,K),
OneMoreMove is K + 1,
retract(stored_answer(moves,_)),
asserta(stored_answer(moves,OneMoreMove)),

% case knowledge base - user responses

:- dynamic(stored_answer/2).

% procedure to get rid of previous responses
% without abolishing the dynamic declaration

clear_stored_answers :- retract(stored_answer(_,_)),fail.
clear_stored_answers.