code: function(or1, or2) {
  			var d1 = set.difference(or1, or2.values[0]);
  			var d2 = set.difference(or1, or2.values[1]);
  			return set.intersection(d1, d2);
  		}
docs: (a ∪ b) \ (c ∪ d) where Z = (a ∪ b) -> Z \ (c ∪ d) -> (Z \ c) ∩ (Z \ d) -> ((a ∪ b) \ c) ∩ ((a ∪ b) \ d)