Added samples

snippets/powerset.md

@@ -5,4 +5,5 @@ Use `reduce()` combined with `map()` to iterate over elements and combine into a
 ```js
 const powerset = arr =>
   arr.reduce( (a,v) => a.concat(a.map( r => [v].concat(r) )), [[]]);
+// powerset([1,2]) -> [[], [1], [2], [2,1]]
 ```