739: Mainline

At this point, your definitions are such that you can construct an empty list and add elements to an existing list, but you cannot get at a list's elements. To get at the elements, you need the following:

• A pointer, current_link_pointer
• A function, advance, that advances current_link_pointer from one link to the next
• A function, access, that obtains a pointer to a railroad_car object from the link pointed at by current_link_pointer
• A function, endp, for end predicate, that determines whether current_link_pointer's value is NULL
• A function, reset, that assigns the value of current_link_pointer to the value of first_link_pointer