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]
+```

snippets/array-intersection.md

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+### Array intersection (Common values between two arrays)
+
+Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values contained in `b`.
+
+```js
+const intersection = (a, b) => { const s = new Set(b); return a.filter(x => s.has(x)); }
+// intersection([1,2,3], [4,3,2]) -> [2,3]
+```

snippets/array-union.md

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+### Array union
+
+Create a `Set` with all values of `a` and `b` and convert to an array.
+
+```js
+const union = (a, b) => Array.from(new Set([...a, ...b]))
+// union([1,2,3], [4,3,2]) -> [1,2,3,4]
+```

snippets/difference-between-arrays.md

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-### Difference between arrays
-
-Use `filter()` to remove values that are part of `values`, determined using `includes()`.
-
-```js
-const difference = (arr, values) => arr.filter(v => !values.includes(v));
-// difference([1,2,3], [1,2]) -> [3]
-```