> now `extract_head` should return the leftmost head directly, using its
> `abort_handler`:
>
-> let extract_head = \lst larger_computation. lst
-> (\hd sofar continue_handler abort_handler. abort_handler hd)
-> junk
-> larger_computation
-> larger_computation
+> let extract_head = \lst larger_computation. lst
+> (\hd sofar continue_handler abort_handler. abort_handler hd)
+> junk
+> larger_computation
+> larger_computation
>
> 3. To extract tails efficiently, too, it'd be nice to fuse the apparatus
> developed in these v5 lists with the ideas from