### powerset

Returns the powerset of a given array of numbers.

Use `Array.reduce()` combined with `Array.map()` to iterate over elements and combine into an array containing all combinations.

```js
const powerset = arr =>
  arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
```

```js
powerset([1,2]) // [[], [1], [2], [2,1]]
```