code: function(and, or) {
  			var aDiff = set.difference(and, or.values[0]);
  			var bDiff = set.difference(and, or.values[1]);
  			return set.intersection(aDiff, bDiff);
  		}
docs: (a ∩ b) \ (c u d) where Z = (a ∩ b) -> Z \ (c u d) -> (Z \ c) ∩ (Z \ d) -> ((a ∩ b) \ c) ∩ ((a ∩ b) \ d)