% From Metagol graph-connectedness.pl

pos(target(a,b)).
pos(target(b,c)).
pos(target(c,d)).
pos(target(b,a)).
pos(target(a,c)).
pos(target(a,d)).
pos(target(a,a)).
pos(target(b,d)).
pos(target(b,a)).
pos(target(b,b)).