text: #include "utils/iterator.h"
  #include <stdlib.h>
  
  void* iter_next(iterator *iter) {
      if(iter->next == NULL) return NULL;
      if(iter->ended) return NULL;
      return iter->next(iter);
  }
  
  void* iter_prev(iterator *iter) {
      if(iter->prev == NULL) return NULL;
      if(iter->ended) return NULL;
      return iter->prev(iter);
  }
  