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