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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 _init_level_set(init_level_set, image_shape):
 """Auxiliary function for initializing level sets with a string. If `init_level_set` is not a string, it is returned as is. """

    if isinstance(init_level_set, str):
        if init_level_set == 'checkerboard':
            res = checkerboard_level_set(image_shape)
        elif init_level_set == 'circle':
            res = circle_level_set(image_shape)
        else:
            raise ValueError("`init_level_set` not in "
                             "['checkerboard', 'circle']")
    else:
        res = init_level_set
    return res