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.

>>251
"Emulating" an iterator us better left to functions.
I don't believe in C standard as some religion. Please avoid treating it as such, it makes arguing about changes to C much more subjective(one likes C standard, one want another C standard, one wants to scrap the entire standard and start over and all of them have their arguments replaced by blind adherence/disapproval of some abstract bureaucratic bullshit)