@define third(x) {
  switch (x) {
  @match [_,_,h,_ ...]: return h;
  default: printf("non-list or length < 3\n"); return -1;
  }
}

printf("got %d\n", third([1,2,3]));
printf("got %d\n", third([1,2,4,8,10]));
third([1,2]);
third("hello");