occupation :: Set (Set (Man, Job))
occupation = Set.filter (invalid [(Clark, Butcher), (Jones, Butcher)]) pairsSet
occupation :: Set (Set (Man, Job))
occupation = Set.filter (invalid [(Clark, Butcher), (Jones, Butcher)]) pairsSet
occupation = Set.filter (invalid [(Clark, Butcher), (Jones, Butcher)]) pairsSet