Just as pedantry (for a kind of algebraic topology of lists?), there's a short exact sequence here: 0 ↪ todolist ↪ checklist ↠ list ↠ 0 f

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