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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 build_extension(extensions: Sequence[ExtensionHeader]) -> str: """ Unparse a ``Sec-WebSocket-Extensions`` header. This is the reverse of :func:`parse_extension`. """

    return ", ".join(
        build_extension_item(name, parameters) for name, parameters in extensions
    )