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<p>Hierarchical keywords metadata defined by the MWG 2.0 specification.  These
tags belong to the ExifTool XMP-mwg-kw group. ExifTool unrolls keyword
structures to an arbitrary depth of 6 to allow individual levels to be
accessed with different tag names, and to avoid infinite recursion.  See
<a href="http://www.metadataworkinggroup.org/">http://www.metadataworkinggroup.org/</a> for the official specification.</p>
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