% generating primes in prolog
% This grows a data base of primes - with the largest prime first...
prime(3).
prime(2).
new_prime(I):-var(I),!, repeat,generate(I), asserta(prime(I)).
new_prime(I):- % nonvariable(I)
	 write('Usage: new_prime(Variable)'), nl, !, fail.

generate(Next):-
	prime(Last), 
	Plus2 is Last +2, !,
	scan(candidate, Plus2), 
	candidate(Next), 
	not has_divisor(Next),!.

has_divisor(J):- prime(D), 0 is  J mod D.

% Scanning the integers scan the next with a property
% use like this: scan(i,First),i(I), Until,abolish(i)...

scan(Label,I):- V=I,
	 (Garbage=..[Label,_],  Garbage,abolish(Label); true),
         Count=..[Label,V], asserta(Count).
scan(Label,I):-Count=..[Label,V], retract(Count,_),
         Next is V+2, Count2=..[Label,Next], asserta(Count2).
scan(Label,I):-abolish(Label), fail.

:-trace [generate, candidate, prime, scan].
:-listing.
:- write('input new_prime(P)? to generate a new prime'), nl.