Factorial and powerset

snippets/powerset.md

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+### Powerset
+
+Use `reduce()` combined with `map()` to iterate over elements and combine into an array containing all combinations. 
+
+```js
+var powerset = arr =>
+  arr.reduce( (a,v) => a.concat(a.map( r => [v].concat(r) )), [[]]);
+```