code: private void extendTail(int count) {
          if (terminated || count < 1) {
              return;
          }
  
          while (--count >= 0) {
              // create a copy of the last slot and make it the last one
              final Slot inserted = slots.insertSlotAt(slots.size(), slots.getLastSlot());
              inserted.setValue(null);
              inserted.withTags(TAG_EXTENSION);
          }
      }
docs: Inserts slots at the and of the mask and mark newly inserted slot as 'extension' (extended
  tail of non-terminated mask). 'Extended' slots will be removed when their values are cleared