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This patch uses:
 tests = self._registries['cross_tests'].values()
to get a list of functions to run at the end of '_parse_known_args'.
I replaced all of the mutually_exclusive_group tests (3 places) in the ArgumentParser with a static function defined in class _MutuallyExclusiveGroup, and registered this function. This refactoring should make it easier to add other specialized groups (e.g. inclusive) in the future.
I'm using the _registries because they are already being shared among groups. 
A user can also register a custom testing function. For example:
 def inclusive_test(parser, seen, *args):
 # require an argument from one of the 2 groups, g0 and g1
 g0seen = seen.intersection(g0._group_actions)
 g1seen = seen.intersection(g1._group_actions)
 if len(g0seen.union(g1seen))==0:
 parser.error('one of the 2 groups is required')
 parser.register('cross_tests','inclusive', inclusive_test)
This patched 'argparse.py' runs 'test_argparse.py' without error.
This patch does not include the issue18943 changes, which make setting 'seen_non_default_actions' more reliable.