}
      /*
       * requireNonNull is safe because:
       *
       * - columnIterator started off pointing to an empty iterator, so we must have entered the
       *   `if` body above at least once. Thus, if we got this far, that `if` body initialized
       *   rowEntry at least once.
       *
       * - The only case in which rowEntry is cleared (during remove() below) happens only if the