What is the idea behind sets of preferences?

The idea is nicely illustrated with the following example dealing with ``car features'':

Consider where in buying a car one ranks the price (expensive) over safety features (safe) over power (powerful), but safety features together with power is ranked over price.

This is expressed in our syntax as follows, taken from file car.lp

expensive :- name(e), not neg expensive.
powerful  :- name(p), not neg powerful.
safe      :- name(s), not neg safe.

% false       :- expensive, powerful, safe.
neg expensive :-            powerful, safe.
neg powerful  :- expensive,           safe.
neg safe      :- expensive, powerful      .

m1 : [p].
m2 : [s].
m3 : [e].

m4 : [p,s].

m1 < m2.
m2 < m3.
m3 < m4.