/usr/lib/mlton/sml/basis/posix/ is in mlton-basis 20130715-3.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/mlton/sml/basis/posix/error.sig
/usr/lib/mlton/sml/basis/posix/error.sml
/usr/lib/mlton/sml/basis/posix/file-sys.sig
/usr/lib/mlton/sml/basis/posix/file-sys.sml
/usr/lib/mlton/sml/basis/posix/flags.sig
/usr/lib/mlton/sml/basis/posix/flags.sml
/usr/lib/mlton/sml/basis/posix/io.sig
/usr/lib/mlton/sml/basis/posix/io.sml
/usr/lib/mlton/sml/basis/posix/posix.sig
/usr/lib/mlton/sml/basis/posix/posix.sml
/usr/lib/mlton/sml/basis/posix/pre-posix.sml
/usr/lib/mlton/sml/basis/posix/proc-env.sig
/usr/lib/mlton/sml/basis/posix/proc-env.sml
/usr/lib/mlton/sml/basis/posix/process.sig
/usr/lib/mlton/sml/basis/posix/process.sml
/usr/lib/mlton/sml/basis/posix/signal.sig
/usr/lib/mlton/sml/basis/posix/signal.sml
/usr/lib/mlton/sml/basis/posix/stub-mingw.sml
/usr/lib/mlton/sml/basis/posix/sys-db.sig
/usr/lib/mlton/sml/basis/posix/sys-db.sml
/usr/lib/mlton/sml/basis/posix/tty.sig
/usr/lib/mlton/sml/basis/posix/tty.sml