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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 set_default_backend(name: str):
 """Choose a default backend."""

    global _default_backend
    if name == "bokeh":
        raise RuntimeError("Support for bokeh has been discontinued. At some point, we may return to support holoviews.")
    if not name in backends:
        raise RuntimeError("Backend {0} is not supported and cannot be set as default.".format(name))
    _default_backend = name