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# can only stack on repositories that have compatible internal
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if getattr(repo._format, 'supports_tree_reference', False):
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matching_format_name = 'rich-root-pack'
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mismatching_format_name = 'pack-0.92'
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mismatching_format_name = 'pack-0.92-subtree'
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base = self.make_repository('base', format=matching_format_name)
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repo.add_fallback_repository(base)