/usr/share/doc/libptscotch-dev/CeCILL-C_V1-fr.txt.gz is in libptscotch-dev 5.1.12b.dfsg-2build1.
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It is a symlink to /usr/share/doc/libptscotch-dev/../libscotch-dev/CeCILL-C_V1-fr.txt.gz