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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 add_signature_block(src_fileobj, dest_fileobj, signing_algorithm, signature=None):
 """Add a signature block to marfile, a MarReader object. Productversion and channel are preserved, but any existing signatures are overwritten. Args: src_fileobj (file object):
 The input MAR file to add a signature to dest_fileobj (file object):
 File object to write new MAR file to. Must be open in w+b mode. signing_algorithm (str):
 One of 'sha1', or 'sha384' signature (bytes):
 Signature to write, or None to use a dummy signature """

    algo_id = {'sha1': 1, 'sha384': 2}[signing_algorithm]
    if not signature:
        signature = make_dummy_signature(algo_id)

    src_fileobj.seek(0)
    mardata = mar.parse_stream(src_fileobj)

    # Header
    header = mardata.header
    dest_fileobj.write(mar_header.build(header))

    # Signature block
    sig = dict(algorithm_id=algo_id,
               size=len(signature),
               signature=signature,
               )

    # This will be fixed up later
    filesize = 0
    sigs_offset = dest_fileobj.tell()
    sigs = sigs_header.build(dict(
        filesize=filesize,
        count=1,
        sigs=[sig],
    ))
    dest_fileobj.write(sigs)

    # Write the additional section
    dest_fileobj.write(extras_header.build(mardata.additional))

    # Write the data
    data_offset = dest_fileobj.tell()
    src_fileobj.seek(mardata.data_offset)
    write_to_file(takeexactly(src_fileobj, mardata.data_length), dest_fileobj)

    # Write the index
    index_offset = dest_fileobj.tell()

    index = mardata.index

    # Adjust the offsets
    data_offset_delta = data_offset - mardata.data_offset

    for e in index.entries:
        e.offset += data_offset_delta

    dest_fileobj.write(index_header.build(index))
    filesize = dest_fileobj.tell()

    # Go back and update the index offset and filesize
    dest_fileobj.seek(0)
    header.index_offset = index_offset
    dest_fileobj.write(mar_header.build(header))

    dest_fileobj.seek(sigs_offset)
    sigs = sigs_header.build(dict(
        filesize=filesize,
        count=1,
        sigs=[sig],
    ))
    dest_fileobj.write(sigs)