Function append_convergence 

pub fn append_convergence(
    path: &Path,
    expected_file_hash: &str,
    items: &[String],
) -> Result<Vec<String>, String>

Appends - [ ] N. Convergence with one open sub-task N.k per item, N after the highest top-level number. The file must still hash to expected_file_hash; the old bytes stay an exact prefix.

Errors

The file changed since convergence began, the append would change an existing task, or the file cannot be written.