if (counts[i] >= 0) {
          sortedCounts[index] += counts[i];
        } else {
          sortedCounts[index] = ~counts[i];
        }
      }
      // Note that we're not getting rid, yet, of elements with count 0.  We'll do that in build().
      this.elements = sortedElements;
      this.counts = sortedCounts;
      this.length = uniques;
    }

    /**

     * We track the "meta iterators," the iterators-of-iterators, below.  Usually, topMetaIterator
     * is the only one in use, but if we encounter nested concatenations, we start a deque of
     * meta-iterators rather than letting the nesting get arbitrarily deep.  This keeps each
     * operation O(1).
     */

    @CheckForNull private Iterator<? extends Iterator<? extends T>> topMetaIterator;