Added symmetric difference as its own snippet, resolve #168

snippets/array-difference.md

@@ -3,14 +3,6 @@
 Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values not contained in `b`.
 
 ```js
-const difference = (a, b) => { const s = new Set(b); a.filter(x => !s.has(x)); };
+const difference = (a, b) => { const s = new Set(b); return a.filter(x => !s.has(x)); };
 // difference([1,2,3], [1,2,4]) -> [3]
 ```
-
-This function can also be used to generate the symmetric difference or disjunctive union by swapping the
-arrays and combining the results.
-
-```js
-const symmetricDifference = (a, b) => [...difference(a,b), ...difference(b,a)];
-// difference([1,2,3], [1,2,4]) -> [3,4]
-```