Just as pedantry (for a kind of algebraic topology of lists?), there's a short exact sequence here:<p><pre><code> 0 ↪ todolist ↪ checklist ↠ list ↠ 0
f g
</code></pre>
`f` takes the active items of a todolist and decorates them as undone [ ] items, yielding a checklist<p>`g` takes a checklist and forgets entire undone [ ] items as well as the [x] of the done items, yielding a list<p>Exercise: does it split?