I believe the point is that IF the problem with nil is that when it shows up you can't be sure whether it was on purpose (to indicate an "empty" value) or on ac

I believe the point is that IF the problem with nil is that when it shows up you can't be sure whether it was on purpose (to indicate an "empty" value) or on accident (because an error in the code caused a variable to not get bound). By introducing a separate identifier (e.g. :id-unknown) to explicitly handle the empty-value case, it frees up nil to solely signify improperly bound variables. [EDIT: Emphasis]