public boolean offerLast(E e) {
      synchronized (mutex) {
        return delegate().offerLast(e);
      }
    }

    @Override
    public E removeFirst() {
      synchronized (mutex) {
        return delegate().removeFirst();
      }
    }

    @Override
    public E removeLast() {
      synchronized (mutex) {
        return delegate().removeLast();
      }
    }

      while (topMetaIterator == null || !topMetaIterator.hasNext()) {
        if (metaIterators != null && !metaIterators.isEmpty()) {
          topMetaIterator = metaIterators.removeFirst();
        } else {
          return null;
        }
      }
      return topMetaIterator;
    }

    @Override
    public boolean hasNext() {

      while (topMetaIterator == null || !topMetaIterator.hasNext()) {
        if (metaIterators != null && !metaIterators.isEmpty()) {
          topMetaIterator = metaIterators.removeFirst();
        } else {
          return null;
        }
      }
      return topMetaIterator;
    }

    @Override
    public boolean hasNext() {