So what you are saying is that a fold is not provided because it would not necessarily be terminating, right? But an unfold may also easily be non-terminating, may it not? And can we not have productive folds, given laziness? Either I am missing something or this is not an applicable reason.
A very nice tutorial. I added it to my reading list. But it is also quite long, and at this time I cannot give it the time and attention it requires. Is there some specific section I should read before proceeding with the present inquiry?