#include <stdio.h>

void walk_stairs_1(staircase s){
  for(step = 1, step <= last_step(s); ++step)
    do_step(step);
}

void walk_stairs_2(staircase s){
  if(!empty_staircase(s)){
     do_step(first_step(s));
     walk_stairs(remaining_steps(s))
  }
}

int main(void) {
  /* ... */
  return 0;
}