@Test public void shouldCalculateEveryCombination() {

  Set<Integer> input = new HashSet<Integer>();
  input.add(1);
  input.add(2);
  Set<Set<Integer>> result = combinationOfIntegers.calculate(input);
  assertSetContainsSet(result);
  assertSetContainsSet(result, 1);
  assertSetContainsSet(result, 2);
  assertSetContainsSet(result, 1, 2);
}