class callable_value : public std::enable_shared_from_this<callable_value>
{
public:
    callable_value(boost::any value, formula_callable* parent)
        : value_(value)
        , parent_(parent)
    {}

    boost:any& get_value() const
    {
        return value_;
    }

    formula_callable* get_parent() const
    {
        return parent_;
    }

private:
    boost::any value_;
    formula_callable* parent_;
};