code: def _make_signed_headers(headers, headers_to_sign):
      
      return b";".join(header.lower() for header in sorted(headers_to_sign)
                       if header in headers)
docs: 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}