let sysf_cons x xs = fun c n -> fun () -> c x (xs c n);;
let sysf_length xs = xs (fun x z -> z () + 1) (fun () -> 0) ();;
-Notice that we have `sysf_cons` returning a `fun () ->`, and also that in `sysf_length` we have to apply a `()` to the seed argument to evaluate it and extract its value, and also we apply a `()` to the output of the `sysf_length` function.
+Notice that we have `sysf_cons` returning a `fun () ->`, and also that in `sysf_length` we have to apply a `()` to the seed argument to evaluate it and extract its value. That's a good thing, we don't *want* to automatically evaluate the fold over the rest of the list if doing so might invoke a `blackhole` or `failwith`. We only want the fold over the rest of the list to be evaluated when we specifically request it, by feeding the thunk a `()` argument. (This is called "forcing the thunk.") Also, after the fold has traversed the whole list, what we'll have at that point will be a thunk (it has to have the same type as the seed value). So we have to force it to get our answer --- that's why there's a `()` at the end of `sysf_length`.
Now how are we doing?