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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 root_block(template_name=DEFAULT_TEMPLATE_NAME):
 """A decorator that is used to define that the decorated block function will be at the root of the block template hierarchy. In the usual case this will be the HTML skeleton of the document, unless the template is used to serve partial HTML rendering for Ajax. The :func:`root_block` decorator accepts the following arguments: :param template_name: The name of the block template hierarchy which is passed to the :func:`render_template` document rendering function. Different templates are useful for rendering documents with differing layouts (e.g. admin back-end vs. site front-end), or for partial HTML rendering for Ajax. """

    def decorator(block_func):
        block = RootBlock(block_func, template_name)
        return block_func
    return decorator