code: public static <E> Set<Set<E>> powerSet(Set<E> set) {
          return new PowerSet<>(set);
      }
docs: Note: copy from Google Guava under Apache License v2.
  <br />
  
  Returns the set of all possible subsets of {@code set}. For example,
  {@code powerSet(ImmutableSet.of(1, 2))} returns the set {@code {{},
  {1}, {2}, {1, 2}}}.
  
  <p>Elements appear in these subsets in the same iteration order as they
  appeared in the input set. The order in which these subsets appear in the
  outer set is undefined. Note that the power set of the empty set is not the
  empty set, but a one-element set containing the empty set.
  
  <p>The returned set and its constituent sets use {@code equals} to decide
  whether two elements are identical, even if the input set uses a different
  concept of equivalence.
  
  <p><i>Performance notes:</i> while the power set of a set with size {@code
  n} is of size {@code 2^n}, its memory usage is only {@code O(n)}. When the
  power set is constructed, the input set is merely copied. Only as the
  power set is iterated are the individual subsets created, and these subsets
  themselves occupy only a small constant amount of memory.
  
  @param set the set of elements to construct a power set from
  @return the power set, as an immutable set of immutable sets
  @throws IllegalArgumentException if {@code set} has more than 30 unique
  elements (causing the power set size to exceed the {@code int} range)
  @throws NullPointerException if {@code set} is or contains {@code null}
  @see <a href="http://en.wikipedia.org/wiki/Power_set">Power set article at
  Wikipedia</a>