code: public <E> Collection<E> disjunction(Collection<E> a, Collection<E> b)
      {
          if (a == null) {
              return b;
          } else if (b == null) {
              return a;
          }
          return CollectionUtils.disjunction(a, b);
      }
docs: Returns a {@link Collection} containing the exclusive disjunction (symmetric difference) of the given
  {@link Collection}s.
  
  @param <E> the type of the elements in the collection
  @param a the first collection, must be non-null
  @param b the second collection, must be non-null
  @return the symmetric difference of the two collections