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.

>>246
Additional reply;
Loop macro is not really needed in C code, and you can abstract over with function calls.
Its not a pretty as map/reduce but you can work at a similar level of abstraction/code density without the requirement of entire high-level bloat layer. Add a couple of function pointers, and you can abstract away at your leisure.
The people who add things like foreach macros are trying to force it the wrong way(its not semantically wrong just lacks native support: i would want such thing supported only at the level of a language, not as a macro i would have to rely on), since they think container-specific functions are unelegant.