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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 slice_pdf(input_filename: str, output_filename: str, slice_horiz: int, slice_vert: int) -> str: """ Slice each page of the original, to convert to "one real page per PDF page". Return the output filename. """

    if slice_horiz == 1 and slice_vert == 1:
        log.debug("No slicing required")
        return input_filename  # nothing to do
    log.info("Slicing each source page mv into {} horizontally x {} vertically",
             slice_horiz, slice_vert)
    log.debug("... from {!r} to {!r}", input_filename, output_filename)
    require(MUTOOL, HELP_MISSING_MUTOOL)
    run([
        MUTOOL,
        "poster",
        "-x", str(slice_horiz),
        "-y", str(slice_vert),
        input_filename,
        output_filename
    ])
    return output_filename