Ambiguous call to variadic template function

I'm creating some classes which represent the functions in the mathematical meaning and their interface is to be 'math-friendly'. To accomplish this I want to create a variadic template operator() methods, which allow user to write defining functions in this way f(x, y) = 2*x + y; and then getting its value by calling f(4, 5);. I cannot predict the number of parameters (the number of variables in mathematical function) so I decided to use variadic templates. However overloading operator() twice as a variadic template and calling it causes the "ambigous call" error. Is there some way to overcome it, or do I have to create two seprate methods?

//expressions like f(x, y, z) = x*x+y-z    
template <typename... Args>
RichFunction<T> &operator()(Args&&... args)
{
        mMainLinears.clear();
        setMainLinears(args...);
        return *this;
}

//expressions like f(5, 4, 3)
template <typename... Args>
T operator()(Args&&... args)
{
    Function<T>::assign(mMainLinears, 0, args...);
    return Function<T>::value();
}