/usr/share/texmf/multex/ is in multex-base 1.0-1.1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/texmf/multex/amstex/
/usr/share/texmf/multex/amstex/config/
/usr/share/texmf/multex/amstex/config/amsmultex.ini
/usr/share/texmf/multex/latex/
/usr/share/texmf/multex/latex/base/
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/usr/share/texmf/multex/latex/base/bvimbv.fd
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/usr/share/texmf/multex/latex/config/
/usr/share/texmf/multex/latex/config/mullatex.ini
/usr/share/texmf/multex/plain/
/usr/share/texmf/multex/plain/base/
/usr/share/texmf/multex/plain/base/mulplain.tex
/usr/share/texmf/multex/plain/base/multexbase.tex
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/usr/share/texmf/multex/plain/config/
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