/usr/share/linuxdoc-tools/mappings/nippon/README is in linuxdoc-tools 0.9.71-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | general -- nippon specific replacements
mapping -- link to ../groff/tr-mapping, this should be the same for both
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