code: public List<Permutation> getLeftTransversal(int index) {
          List<Permutation> traversal = new ArrayList<Permutation>();
          for (int subIndex = 0; subIndex < size; subIndex++) {
              if (permutations[index][subIndex] != null) {
                  traversal.add(permutations[index][subIndex]);
              }
          }
          return traversal;
      }
docs: Get the traversal U<sub>i</sub> from the list of transversals.
  
  @param index the index of the transversal
  @return a list of permutations