Executable XML users challenge

provide any LISP macro with clear explanation of its structure and function
that is :
1.concise (max 10 lines)
2.does not have a C equivalent(at least not anything above 100 lines)
3.does not use any libraries or imported complex functions which are not in C
If you provide an exact explanation/commentary on what it does i'll try to make a C solution which
is equivalent to LISP one. If i fail to do so, LISP wins, if i provide a solution you can make another macro.
If all examples in the thread are provide with C equivalents, LISP loses.

>>38
#define square(x) ((x) * (x))

square(rand() % 10) expands to ((rand() % 10) * (rand() % 10)). The macro is for when you only want the argument to be evaluated once, and then you use the returned value in the body of the macro.