You should somehow express that A is connected to B by 2 links if there is a C such that A is connected to C directly and C is connected to B directly. So you need to create an additional rule called 'link2: Microsoft: SNA / Host Integration Server Links' or something appropriate to express 'connected by 2 direct links'

Also, take care of cycles. You'll see that a naive approach to this problem would report each city as being connected to itself by 2 links, because A - B and B - A also fulfill the rule above.

hi

I need one sample of the output that you want
ie,one virtual result of the program.

thanks

link(X,C),link(C,Y),X \= Y.

link(detroit,toronto).
link(chicago,washington).
link(denver,sacramento).
link(sacramento,chicago).
link(toronto,chicago).


connected(X,Z):-link(X,Y),link(Y,Z).
%connected(X,Z):-link(Z,Y),link(Y,X).

cities(X,Y,C):-findall([X,Y],connected(X,Y),C).