/usr/include/flint/flintxx/ is in libflint-dev 2.5.2-17build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/flint/flintxx/default_rules.h
/usr/include/flint/flintxx/evaluation_tools.h
/usr/include/flint/flintxx/expression.h
/usr/include/flint/flintxx/expression_traits.h
/usr/include/flint/flintxx/flint_classes.h
/usr/include/flint/flintxx/flint_exception.h
/usr/include/flint/flintxx/forwarding.h
/usr/include/flint/flintxx/frandxx.h
/usr/include/flint/flintxx/ltuple.h
/usr/include/flint/flintxx/matrix.h
/usr/include/flint/flintxx/mp.h
/usr/include/flint/flintxx/rules.h
/usr/include/flint/flintxx/stdmath.h
/usr/include/flint/flintxx/traits.h
/usr/include/flint/flintxx/traits_fwd.h
/usr/include/flint/flintxx/tuple.h
/usr/include/flint/flintxx/vector.h