Method operator[]

tools/flatbuffers_backed_impl.cc:46–50

      : list_(list) {}
  int size() const override { return list_ ? list_->size() : 0; }
  CelValue operator[](int index) const override {
    auto value = list_->Get(index);
    return CelValue::CreateStringView(
        absl::string_view(value->c_str(), value->size()));
  }

 private:
  const flatbuffers::Vector<flatbuffers::Offset<flatbuffers::String>>* list_;