## Set of All Subsets

Given an array of items, create a set of all subsets of the items. The result is also called a powerset.

### Function Implementation

```java
Set allSubsets(Array items) {
    if (items == null || items.length == 0) {
        Set s = new Set();
        s.put(new Set());
        return s;
    }

Item item = items.removeFirst();
    Set sets = allSubsets(items);
    Set newSets = sets.copy();

for (set in sets) {
        set.put(item);
        newSets.put(set);
    }

return newSets;
}
```