Hi, I’m having troubles implementing this F# snippet: fast F# permutations *(tested it, works ok)*

## My code so far:

```
/* jshint esversion: 6 */
const
isEmpty = x => x.length === 0,
cons = (h, t) => [h].concat(t);
function insertions(x, ys) {
const [head, ...tail] = ys;
return isEmpty(ys) ? [[x]] :
cons(cons(x, ys), insertions(x, tail).map(x => cons(head, x)));
}
function permutations(xs) {
const [head, ...tail] = xs;
return isEmpty(xs) ? [[]] :
permutations(tail).map(ys => insertions(head, ys));
}
```

repl.it ->

It seems that *permutations* function generates something close to that I need, but formatted in wrong way.

UPD: *Found correct solution:*

```
const insertions = (x, [y, ...ys]) =>
!y ? [[x]] :
[[x, y, ...ys], ...insertions(x, ys).map(zs => [y, ...zs])];
const permutations = ([x, ...xs]) =>
!x ? [[]] :
[].concat(...permutations(xs).map(ys => insertions(x, ys)));
```