This is a small sheet that reviews list operations.  We'll be covering
the following:

    car, cdr, list, cons, append

For all these examples, assume that 'L' stands for a list of some
kind.  'E' is an element --- it can be anything (list, number,
symbol).

---
car
---
usage: (car L)
description:  This returns the first element of a list.
examples:
    (car '(this is a test)) => 'this
    (car '(1)) => 1
    (car '()) => error --- we can't take the car of an empty list.
    (car '((this is) nested)) => '(this is) --- lists are also
                                  elements.

---
cdr
---
usage: (cdr L)
description:  This returns the rest of the list.  What we mean by the
              "rest" is the list with the first element taken out.
examples:
    (cdr '(this is a test)) => '(is a test)  --- cross out 'this.
    (cdr '(1)) => '()  --- cross out 1.
    (cdr '()) => error --- we can't take the cdr of an empty list
                 either.
    (cdr '((this is) nested)) => '(nested) --- cross out the first
                                           element '(this is)



---
list
---
usage: (list E1 E2 ... )  --- element can be anything.
       Can take any number of arguments.
description: Creates a list with elements: E1, E2, ...,
             and so on.
hint: Here's a procedure for getting list right.  Suppose that we're
trying out:

    (list 1 2 3 '4 '(five))

    1.  Write down all the arguments in a line, taking out all quotes
    
        1 2 3 4 (five)

    2.  Put parentheses around the whole thing, and begin it with a quote.

        '(1 2 3 4 (five))

    3.  That's it.  This is a list of 5 elements.

examples:
    (list 'this 'is 'a 'test) => '(this is a test)
    (list '(1)) => '((1)) --- note: you CAN make a list of one element.
    (list '()) => '(())
    (list (list 'this 'is) 'nested)
        => (list '(this is) 'nested)
        => '((this is) nested)

---
cons
---
usage: (cons E L)
description: Attach element E to the front of the list L and return
             that list.
hint:  Let's try:  (cons 1 '(2)).
    
    1.  Copy down the arguments to cons, taking out the quotes.

        1 (2)

    2.  Next, move the left parens of the second argument to enclose
        the first.

        1 (2)
          ^------  move this parenthesis left
         
        (1 2)

    3.  Add a quote in the front, and we're done.

        '(1 2)

examples:
    (cons 'hello '(world)) => '(hello world)
    (cons '(alpha) '(beta)) => '((alpha) beta)
    (cons 'slashdot '((kuro5hin kenshin) (cowboy bebop)))
        => '(slashdot (kuro5hin kenshin) (cowboy bebop))

---
append
---
usage: (append L1 L2 ... LN)
description: Take the elements of L1, L2, ... LN, and put them all
             into a single list.
hint: Let's try (append '(1 one) '(2 (two)) '((3) three)):

    1.  Write out L1, L2, ... LN, each without the outermost
        parentheses:

        1 one 2 (two) (3) three

    2.  Put paretheses around the whole thing, and top it with a
        quote.

        '(1 one 2 (two) (3) three)

examples:

    (append '(zerg) '(vs) '(protoss)) => '(zerg vs protoss)
    (append '(Harkonnen) '((vs)) '(Atreides)) 
        => '(Harkonnen (vs) Atreides)
    (append '() '()) => '()  --- yes, this is silly, but it works.
    (append '(unix) (list 'dos 'threads))
        => (append '(unix) '(dos threads))
        => '(unix dos threads)