Function tick 

pub fn tick(
    path: &Path,
    task_id: &str,
    expected_line_hash: &str,
) -> Result<(), String>

Ticks task_id: x into its [ ]; and into its parent’s when every required sibling is ticked now. Nothing else changes.

Errors

The task is not open, its line changed since it was read, or the file cannot be written.