% From murphy Thu Oct 31 14:53 PST 1991
% To: dick
% Subject: complexity of prolog
%     While going through some of my old files, I found a short
%prolog program you may find interesting. The Air Force was
%considering the development of a massively parallel prolog 
%interpreter for the SDI program until I showed them this one.

num(a,a,a,a).
num(X1,X2,X3,b):-num(X1,X2,X3,a).
num(X1,X2,b,a):-num(X1,X2,a,b).
num(X1,b,a,a):-num(X1,a,b,b).
num(b,a,a,a):-num(a,b,b,b).

%What does it do? Ask it trace, num(b,b,b,b).