. "For example, the bidirectional complement rule A<->B indicates that A is a complement of B and B is a complement of A. This is a more general, and less powerful mechanism than unidirectional rules, since every rule X<->Y can be transformed into two rules: X???Y and Y???X. However, there is no bidirectional way to represent that most ???Diet Cereal??? customers buy skim milk, but relatively few ski" . . .