Schemes:
  SK(A,B)
Facts:
  SK('a','c').
  SK('b','c').
  SK('b','b').
  SK('b','c').
Rules:
  DoNothing(Z) :- Stuff(Z).
Queries:
  SK(A,'c')?
  SK('b','c')?
  SK(X,X)?
  SK(A,B)?