Method add

include/pybind11/pytypes.h:2346–2348

    }
    template <typename T>
    bool add(T &&val) /* py-non-const */ {
        return PySet_Add(m_ptr, detail::object_or_cast(std::forward<T>(val)).ptr()) == 0;
    }
    void clear() /* py-non-const */ { PySet_Clear(m_ptr); }
};