Navigate the zipper to the first element
in the structure matching predfn. A linear scan
is done using NEXT to find the element.

Navigate to the empty subsequence directly to the
left of this element.

Navigate to the empty subsequence directly to the
right of this element.

Navigate to the element to the left.
If no element there, works like STOP.

Navigate to the next element in the structure.
If no next element, works like STOP.

Navigate to every element reachable using calls
to NEXT

Navigate to the subsequence containing only
the node currently pointed to. This works just
like srange and can be used to remove elements
from the structure

Navigate to the previous element.
If this is the first element, works like STOP.

Navigate to the element to the right.
If no element there, works like STOP.