Function keep_bytes 

pub fn keep_bytes(
    root: &Path,
    part: &str,
    file: &str,
    bytes: &[u8],
    now_ms: i64,
) -> Result<PathBuf, String>

Bytes kept as a version of file (a calendar’s item set aside, ACCOUNTS), as keep keeps a file: under a hidden name, then renamed. Returns where.