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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 block_to_svg(block=None):
 """ Return an SVG for the block. """

    block = working_block(block)
    try:
        from graphviz import Source
        return Source(block_to_graphviz_string())._repr_svg_()
    except ImportError:
        raise PyrtlError('need graphviz installed (try "pip install graphviz")')