:- constraints leq/2.

leq(X,X)              <=> true.
leq(N1,N2)            <=> number(N1), number(N2) | N1 =< N2.
leq(N1,X) \ leq(N2,X) <=> number(N1), number(N2), N1 > N2 | true.
leq(X,N1) \ leq(X,N2) <=> number(N1), number(N2), N1 < N2 | true.
leq(X,Y)  \ leq(X,Y)  <=> true.
leq(X,Y)  , leq(Y,Z)  ==> leq(X,Z).

truckload(0,0,_,_).
truckload(I,W,D,T) :-                   % do not include pack I
	I > 0,
	I1 is I - 1,
	truckload(I1,W,D,T).
truckload(I,W,D,T) :-                   % include pack I
	I > 0,
	pack(I,Wi,D,Ti),
	W1 is W - Wi,
	W1 >= 0,
	I1 is I - 1,
	truckload(I1,W1,D,T), T = Ti.

pack(30,29,chicago,T) :- leq(19,T),leq(T,29).
pack(29,82,chicago,T) :- leq(20,T),leq(T,29).
pack(28,24,chicago,T) :- leq(8,T),leq(T,12).
%...
pack(3,60,chicago,T) :- leq(4,T),leq(T,29).
pack(2,82,chicago,T) :- leq(28,T),leq(T,29).
pack(1,41,chicago,T) :- leq(27,T),leq(T,28).