add expr_to_float64{_opt}

  return expr_tov<std::string>(expr, bind(&expr_to_string, _1, _2), values);

}

ReturnCode expr_to_float64(

    const Expr* expr,

    double* value) {

  if (!expr_is_value(expr)) {

    return ReturnCode::error("EARG", "expected value");

  }

  try {

    *value = std::stod(expr_get_value(expr));

  } catch (... ) {

    return ReturnCode::errorf("EARG", "invalid number: $0", expr_get_value(expr));

  }

  return OK;

}

ReturnCode expr_to_float64_opt(

    const Expr* expr,

    std::optional<double>* value) {

  double v;

  if (auto rc = expr_to_float64(expr, &v); !rc) {

    return rc;

  }

  *value = v;

  return OK;

}

ReturnCode expr_to_measure(

    const Expr* expr,

    Measure* value) {

    const Expr* prop,

    std::vector<std::string>* values);

ReturnCode expr_to_float64(

    const Expr* prop,

    double* value);

ReturnCode expr_to_float64_opt(

    const Expr* prop,

    std::optional<double>* value);

ReturnCode expr_to_measure(

    const Expr* expr,

    Measure* value);