\begin{equation} 
\phi_i = \sum_{S \subseteq F \setminus \{i\}}  \frac{\vert S \vert ! (\vert F \vert - \vert S \vert - 1)!}{\vert F \vert !} 
    [f_{S \cup \{i\}} (x_{S \cup \{i\}}) - f_S(x_S)]
\end{equation}
