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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def lookup_model_by_kind(kind):
 """Look up the model instance for a given Datastore kind. Parameters: kind(str) Raises: RuntimeError: If a model for the given kind has not been defined. Returns: model: The model class. """

    model = _known_models.get(kind)
    if model is None:
        raise RuntimeError(f"Model for kind {kind!r} not found.")
    return model