Using letters, parentheses, brackets, braces, and the symbols that
we have, we can create expressions that mean something, but we can also
create expressions that don't mean anything.  If we let P mean "Plastic Sam
killed Woozy Wunks" and Q mean "Batboy killed Rubin," then (P -> Q) means
something.  An expression such as (-> P Q) means nothing, though.  We shall
use the term *well-formed formula* to refer to a meaningful and
grammatically correct symbolic expression.  We have three rules that tell
us what a well-formed formula is:

       I.   Any single variable, such as P, TT, ZR, F4, etc., is a
            well-formed formula.

      II.   Using P to represent any symbolic expression, if P is a
            well-formed formula, then so is ~P.

     III.   Using P and Q to represent any two symbolic expressions, if P
            and Q are both well-formed formulas then so are

                (P -> Q)
                (P & Q)
                (P v Q)
                (P <-> Q)

           A.   For the sake of readability, brackets [] and braces {}
                may be used in place of parentheses.

           B.   The outermost parentheses, brackets, or braces may be
                dropped.