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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 _make_signed_headers(headers, headers_to_sign):
 """ Return a semicolon-delimited list of headers to sign. @param headers: The request headers. @type headers: L{dict} @param headers_to_sign: A sequence of header names that should be signed. @type headers_to_sign: L{bytes} @return: The semicolon-delimited list of headers. @rtype: L{bytes} """

    return b";".join(header.lower() for header in sorted(headers_to_sign)
                     if header in headers)