### symmetricDifference*(set1, set2[, ...setN])* The symmetric of two sets is is the set of elements in either set, but not their intersection. If two sets are equal, the symmetric difference is empty.
function(){
		return underscore.reduce(arguments, function(first, second){
			return underscore.union(
				underscore.difference(first, second),
				underscore.difference(second, first)
			);
		});
	}