if (hasNext()) {
        toRemove = iterator;
        return iterator.next();
      } else {
        throw new NoSuchElementException();
      }
    }

    @Override
    public void remove() {
      if (toRemove == null) {
        throw new IllegalStateException("no calls to next() since the last call to remove()");
      }
      toRemove.remove();
      toRemove = null;
    }