while (itr1.hasNext()) {
              E elem1 = itr1.next();
              if (!set2.contains(elem1)) {
                return elem1;
              }
            }
            while (itr2.hasNext()) {
              E elem2 = itr2.next();
              if (!set1.contains(elem2)) {
                return elem2;
              }
            }
            return endOfData();
          }

            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }

        private <T> Set<T> concat(Set<T> set, T elem) {
            Set<T> newSet = new HashSet<>(set);
            newSet.add(elem);
            return newSet;
        }

        @SuppressWarnings("checkstyle:parameternumber")
        private InterimResult build(