% map date to weekday (mon =1, ..., sun = 7);
% The tour starts on Monday. 

 weekday(1,1).
 weekday(D,W) :- D=D1+1, W=W1+1, weekday(D1,W1), W1 < 7.
 weekday(D,1) :- D=D1+1, weekday(D1,7).

% connections with default costs (capitols of the Austrian federal states).

 conn(brg,ibk,2).
 conn(ibk,sbg,2).
 conn(ibk,vie,5).
 conn(ibk,kla,3).
 conn(sbg,kla,2).
 conn(sbg,gra,2).
 conn(sbg,lin,1).
 conn(sbg,vie,3).
 conn(kla,gra,2).
 conn(lin,stp,1).
 conn(lin,vie,2).
 conn(lin,gra,2).
 conn(gra,vie,2).
 conn(gra,eis,1).
 conn(stp,vie,1).
 conn(eis,vie,1).
 conn(stp,eis,2).

conn(B,A,C) :- conn(A,B,C).

city(T) :- conn(T,_,_).

% costing: use default cost, if there are no extra costs 
%

cost(A,B,W,C) :- conn(A,B,C), #int(W), 0 < W, W <= 7, 
                 not ecost(A,B,W).  
ecost(A,B,W) :- cost(A,B,W,C), conn(A,B,C1), C != C1.

% Some example of an extra cost.

cost(stp,eis,2,10).
%cost(eis,stp,2,10).

% ATTENTION: default costs are symmetric, ecosts are NOT!
% The following rule does not suffice to guarantee symetrical
% ecosts but caused bogus results:
%
% ecost(A,B,W) :- ecost(B,A,W).