Function open_mark 

pub fn open_mark(data_set: &Path) -> PathBuf

The file beside a VSAM data set that marks it open for output, as the catalog’s open-for-output indicator does: an OPEN OUTPUT, I-O or EXTEND makes it and the CLOSE after it removes it, so a data set a run left open keeps it for the next OPEN to verify (numeric::assumptions::VSAM_DATA_SET_LEFT_OPEN).