code: @Nonnull
      public static <E> Set<E> union(@Nullable final Set<E> pColl1, @Nullable final Set<E> pColl2)
      {
          final Set<E> result = new HashSet<>();
          if (pColl1 != null) {
              result.addAll(pColl1);
          }
          if (pColl2 != null) {
              result.addAll(pColl2);
          }
          return Collections.unmodifiableSet(result);
      }
docs: Creates a new immutable {@link HashSet} which contains a union of the two given sets.
  
  @param pColl1 first set
  @param pColl2 second set
  @param <E> type of all contained elements
  @return union set