Let \( f(x) = \frac{x - 1}{x + 1} ,\; x \in \mathbb{R} - \{ 0, -1, 1 \} \). If \( f^{\,n+1}(x) = f\!\left(f^{\,n}(x)\right) \) for all \( n \in \mathbb{N} \), then \( f^{6}(6) + f^{7}(7) \) is equal to:
\( \frac{7}{6} \)
\( -\frac{3}{2} \)
\( \frac{7}{12} \)
\( -\frac{11}{12} \)