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.

>>212
Have you used SLIME+Emacs macro expansion key-chords? Also, some implementations allow stepping into macros. Some macros can be a pain to debug, especially those which generate code which generates code (think of once-only as the classical example), but the average garden variety macro isn't that bad.