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.

>>16

(sort '(6 5 8 1 10 4) #'<)
(sort '(6 5 8 1 10 4) #'>)


#include <stdio.h>
#include <stdlib.h>

int comparator(const void *num1, const void *num2) {
    return *(const int *)num1 - *(const int *)num2;
}

int comparator2(const void *num1, const void *num2) {
    return *(const int *)num2 - *(const int *)num1;
}

int main(int argn, char *args[]){
  int arr1[] = {6, 5, 8, 1, 10, 4};
  int arr2[] = {6, 5, 8, 1, 10, 4};
  int i;
  qsort(arr1, sizeof(arr1)/sizeof(int), sizeof(int), comparator);
  for(i=0; i<(int)(sizeof(arr1)/sizeof(int)); i++) printf("%d ", arr1[i]);
  printf("\n");
  qsort(arr2, sizeof(arr2)/sizeof(int), sizeof(int), comparator2);
  for(i=0; i<(int)(sizeof(arr2)/sizeof(int)); i++) printf("%d ", arr2[i]);

  return 0;
}