Learning can_rich and cyclic: experimental results.
1. can_reach(X1,X2) =. true :- linked_to(X1,X2) =. true.
2. can_reach(X1,X2) =. true :- can_reach(X3,X2) =. true,can_reach(X1,X3) =. true.
3. cyclic(X1) =. true :- node_of(X1,X2) =. true,can_reach(X2,X2) =. true.