% isp(P) :- P is a prime number
%    (integer) (+)

% the =:= operator evaluates both sides, and tests for 
% numerical equality
%  1+4 =:= 3+2 
%    will work like this
%  comp(M,N) :- L is M, R is N, L=R.
%  comp(1+4,2+3).
%
% the =\= operator is similar, but tests for inequality

isp(2).
isp(3).
isp(P) :- integer(P), P > 3, P mod 2 =\= 0, \+ has_factor(P,3).  

% has_factor(N,L) :- N has an odd factor F >= L.
%    (integer, integer) (+,+)

has_factor(N,L) :- N mod L =:= 0.
has_factor(N,L) :- L*L<N, L2 is L+2, has_factor(N,L2).