code: public void checkDefaultGroupSequenceIsExpandable(final List<Class<?>> defaultGroupSequence)
        throws GroupDefinitionException {
      for (final Map.Entry<Class<?>, List<Group>> entry : sequenceMap.entrySet()) {
        final Class<?> sequence = entry.getKey();
        final List<Group> groups = entry.getValue();
        final List<Group> defaultGroupList = buildTempGroupList(defaultGroupSequence, sequence);
        final int defaultGroupIndex = containsDefaultGroupAtIndex(sequence, groups);
        if (defaultGroupIndex != -1) {
          ensureDefaultGroupSequenceIsExpandable(groups, defaultGroupList, defaultGroupIndex);
        }
      }
    }
docs: check if default group sequence is expandable.
  
  @param defaultGroupSequence list of group classes
  @throws GroupDefinitionException if it's no default group