Add set operations. Union, intersect and difference

snippets/array-difference.md

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+### Array difference (complement)
+
+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); return a.filter(x => !s.has(x)); }
+// difference([1,2,3], [1,2]) -> [3]
+```