The access variable exav is made to point to the first node of the list exlist, irrespective of its previous contents.
The access variable exav is changed to point to the successor of the node it pointed to originally. (Note that this assumes that it pointed to a node originally).
What happens if we:
a) accessfirst(exlist, exav) and the list is empty?
b) accessnext(exlist, exav) when exav points to the last node?
In both cases exav becomes invalid. We could define a test function, eg: validlist(exlist, exav) which will return the value false if the list is empty. This allows us to test for an empty list. We will make use of a function like this in algorithms later.