void indent() {
    for (int i = 0; i < indent_level; ++i) s.append(indent_str);
  }

  // Adjust indention for fields in sequences.

  void StartSequence() override {
    s += "{";
    s += d;
    ++indent_level;
  }

  void EndSequence() override {
    s += d;
    --indent_level;
    indent();
    s += "}";
  }