% P31 (**) Determine whether a given integer number is prime. 

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

is_prime(2).
is_prime(3).
is_prime(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).