map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expected_val, map_val_t new_val);
map_val_t sl_lookup (skiplist_t *sl, map_key_t key);
map_val_t sl_remove (skiplist_t *sl, map_key_t key);
-map_val_t sl_count (skiplist_t *sl);
+size_t sl_count (skiplist_t *sl);
void sl_print (skiplist_t *sl);
void sl_free (skiplist_t *sl);
map_key_t sl_min_key (skiplist_t *sl);