Possible arguments: (none), p7zip,
      partial
Extract using 7z(1) instead of bsdtar(1) and sets
      EXTRACT_SUFX=.7z.  The
      p7zip option forces a dependency on the
      7z from archivers/p7zip if the one from the base
      system is not able to extract the files.
      EXTRACT_SUFX is not changed if the
      partial option is used, this can be used if
      the main distribution file does not have a
      .7z extension.
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