fun <T : OptionGroup> T.cooccurring(): CoOccurringOptionGroup<T, T?>
Make this group a co-occurring group.
The group becomes nullable. At least one option in the group must be required. Of none of the options in the group are given on the command line, the group is null and none of the
required constraints are enforced. If any option in the group is given, all
required options in the group must be given as well.