ancestorStack.push(next);
          }
          // TODO(b/192579700): Use a ternary once it no longer confuses our nullness checker.
          if (!ancestorStack.isEmpty()) {
            return ancestorStack.pop();
          }
          return endOfData();
        }
      };
    }

    /**
     * Visits the next node from the top iterator of {@code horizon} and returns the visited node.