Exercise 13.13

• Make a two-element vector and use set-start! and set-tail! to set the start and tail to empty
• Use an empty list to signify empty

• Use null? to test start or tail for emptiness.

• Check for emptiness
• Return the item (not the node containing the item) at the start

• Use mcons to create a new node for the new item
• What will the mcdr of the new node have to be?
• Set the tail of the queue to the new node, but be sure to save the old tail first
• Make the new node be the tail of the queue
• Make the new node follow the old tail (but what if we are adding to an empty queue?)

• Check for emptiness
• Make the new start of the queue be whatever followed the old start of the queue