code: public static Partition unit(int size) {
          Partition unit = new Partition();
          unit.cells.add(new TreeSet<Integer>());
          for (int i = 0; i < size; i++) {
              unit.cells.get(0).add(i);
          }
          return unit;
      }
docs: Create a unit partition - in other words, the coarsest possible partition
  where all the elements are in one cell.
  
  @param size the number of elements
  @return a new Partition with one cell containing all the elements