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.

>>179
code is text

>>179
1 is the case in >>178. The actual atom/elements of the macro are reversed before being passed to the compiler. In C, you need a lexer/parser to manipulate the AST like that. It's kind of an artificial example which has little use in practice, but I've written macros which did quite fancy code manipulations which are just not possible in text-substitution macro systems. The only way they would be possible is if you could hook/extend your C compiler within the actual program you're coding.