In my last message, I mentioned two variables with slightly odd names:
*print-pretty* and *Self-Eval*. "*" is a perfectly legal part of a
variable name in Lisp, and the convention is that you put "*"s around
GLOBAL variable names in order to make them stand out in your code.
Lisp does not enforce this; it is just a programmer convention. But it
is a widespread convention, so do try to use *Foo* for the names of
global variables, and just Foo for local variables. If you declare
global variables with DEFVAR before setting them, the compiler will
not warn you about using undeclared global variables. 

A similar convention is to put "+" around constants. Constants are
declared via DEFCONSTANT, cannot be changed at run-time, and must be
declared before they are used, since Lisp replaces all the occurrences
with the value. You may never use them this semester, but just in case
you are interested:

(defconstant +Half-Pi+ (/ pi 2.0))

or (with an optional documentation string)

(defconstant +Half-Pi+ (/ pi 2.0)
  "Pi/2, to avoid doing this division repeatedly in my trig routines")