/usr/lib/ocaml/vmthreads/stm.cmi is in libcothreads-ocaml-dev 0.10-3build5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ocaml/vmthreads/../stm.cmi
/usr/lib/ocaml/vmthreads/../stm.cmi