Exercise 1: Implement stacks as linked lists, where null is the empty linked
    list, and the sequence (x,y) serves to hold the first element, x, and the
    remainder of the list, y.  See

        https://en.wikipedia.org/wiki/Stack_(abstract_data_type)

Exercise 2: Implement queues using a mutable list to store the entries,
    cyclically; see

        https://en.wikipedia.org/wiki/Queue_(abstract_data_type)#Queue_implementation

    for the idea.  Be sure to amortize the cost of adding elements to the
    mutable list by at least doubling the length of the list each time the list
    is lengthened.

Exercise 3: Write code to add a border of asterisks to a net.  For example,
	  from "abc" we get this:

	  *****
	  *abc*
	  *****

Exercise 4: Implement a new type of list of length 2, where the first entry is an
    integer called the priority of the second entry, which can be anything.
    Define comparison x?y for such objects to compare the corresponding
    priorities.

Exercise 5: implement priority queues.  See

	https://en.wikipedia.org/wiki/Priority_queue

Exercise 6: implement red black trees, as a way of maintaining lists of objects
    in sorted order.  Follow, for example, the functional approach here:

	https://course.ccs.neu.edu/cs5010/Inheritance/jfp99redblack.pdf
	https://github.com/bmeurer/ocaml-rbtrees

    Use the comparison operator x?y for comparison.