Comment by RodgerTheGreat

There's a simple iterative alternative for generating power sets. For each item of a set, you will either include it or exclude it in one of the results. If you count from 0 to 2^n, the binary digits of your counter will enumerate every combination of these possibilities. In Java 8:

    static <T> Set<Set<T>> power(List<T> a) {
      return IntStream.range(0, 1 << a.size())
      .mapToObj(x -> IntStream.range(0, a.size())
        .filter(y -> (1 << y & x) != 0)
        .mapToObj(y -> a.get(y))
        .collect(Collectors.toSet())
      ).collect(Collectors.toSet());
    }

In JS/ES6:

    function power(a) {
      var r = [];
      for(var x = (1 << a.length) - 1; x >= 0; x--) {
        r.push(a.filter((y, i) => 1 << i & x));
      }
      return r;
    }

In K6:

    {x@&:'+!(#x)#2}