Subsets

Write `subsets(nums)` returning every subset of a list of distinct integers. Order does not matter, but no subset may repeat. subsets([1,2,3]) -> 8 subsets, including [] and [1,2,3] At each index you either take the element or you do not, which is exactly why there are 2^n results. Append a *copy* of the current selection — appending the working list itself gives you n identical empty lists at the end.