Method generate

tests/unit/sys/test_expressionparser.cxx:52–54

    return std::make_shared<BinaryGenerator>(args.front(), args.back());
  }
  BoutReal generate(const Context& ctx) override {
    return a->generate(ctx) + b->generate(ctx);
  }
  std::string str() const override {
    return std::string{"add(" + a->str() + ", " + b->str() + ")"};
  }