expensive(chevrolet).
safe(chevrolet).
safe(volvo).
nice(porsche).
fast(porsche).

neg buy(X) :- name(e(X)), not     buy(X), expensive(X).	% (1)
    buy(X) :- name(s(X)), not neg buy(X), safe(X).	% (2)
    buy(X) :- name(n(X)), not neg buy(X), nice(X).	% (3)
    buy(X) :- name(f(X)), not neg buy(X), fast(X).	% (4)

neg buy(Y) :- name(sp(X,Y)), safe(X), buy(X), neq(X,Y).	% (2')
neg buy(Y) :- name(np(X,Y)), nice(X), buy(X), neq(X,Y).	% (3')
neg buy(Y) :- name(fp(X,Y)), fast(X), buy(X), neq(X,Y).	% (4')

    buy(X) :- name(x1(X)),				% (1.5)
		not neg buy(X),
		not neg safe(X), nice(X), fast(X).
neg buy(Y) :- 	name(x2(X,Y)),				% (1.5')
		not neg safe(X), nice(X), fast(X), 
		buy(X), neq(X,Y).

  x1(X) < e(X1).
x2(X,Y) < e(X1).
   s(X) < x1(X1).
sp(X,Y) < x1(X1).
   s(X) < x2(X1,Y1).
sp(X,Y) < x2(X1,Y1).
   n(X) < s(X1).
np(X,Y) < s(X1).
   n(X) < sp(X1,Y1).
np(X,Y) < sp(X1,Y1).
   f(X) < n(X1).
fp(X,Y) < n(X1).
   f(X) < np(X1,Y1).
fp(X,Y) < np(X1,Y1).

neq(chevrolet,volvo).
neq(chevrolet,porsche).
neq(volvo,chevrolet).
neq(volvo,porsche).
neq(porsche,chevrolet).
neq(porsche,volvo).