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@ -52,7 +52,8 @@ class Relation:
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An empty relation needs a header, and can be filled using the insert()
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method.
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'''
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__hash__ = None # type: None
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def __hash__(self):
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raise NotImplementedError()
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def __init__(self, filename: Optional[Union[str, Path]] = None) -> None:
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self._readonly = False
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