Suppose we would like to define a Lisp construct to increment a
variable value, much like the ++ operator in C. We would like to
write (inc x) and have the effect of (setq x (1+ x)).
Here's a macro definition that does the job:
(defmacro inc (var)
(list 'setq var (list '1+ var)))
When this is called with (inc x), the argument var is the
symbol x---not the value of x, as it would
be in a function. The body of the macro uses this to construct the
expansion, which is (setq x (1+ x)). Once the macro definition
returns this expansion, Lisp proceeds to evaluate it, thus incrementing
x.
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