/usr/include/gecode/set/rel/ is in libgecode-dev 3.7.1-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gecode/set/rel-op/common.hpp
/usr/include/gecode/set/rel-op/inter.hpp
/usr/include/gecode/set/rel-op/partition.hpp
/usr/include/gecode/set/rel-op/post.hpp
/usr/include/gecode/set/rel-op/subofunion.hpp
/usr/include/gecode/set/rel-op/superofinter.hpp
/usr/include/gecode/set/rel-op/union.hpp
/usr/include/gecode/set/rel/common.hpp
/usr/include/gecode/set/rel/eq.hpp
/usr/include/gecode/set/rel/lq.hpp
/usr/include/gecode/set/rel/nosubset.hpp
/usr/include/gecode/set/rel/nq.hpp
/usr/include/gecode/set/rel/re-eq.hpp
/usr/include/gecode/set/rel/re-lq.hpp
/usr/include/gecode/set/rel/re-subset.hpp
/usr/include/gecode/set/rel/subset.hpp