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.

>>155
When i need a lexer to write macros, i'm actually writing a complex code-generation script inside C, which a nested design of layered architectures: this signals something deeply wrong in design.
Yet this makes certain kinds of code much easier to write in Lisp since you don't need a lexer/parser, and much harder to write in C.
The generalized example would be how easy it is to write DSLs in CL, and how you need a whole lexer/parser to do them properly in C (and they may be slower if you're interpreting instead of compiling, which is again something that comes for free in Lisp).