For a suitably chosen real constant a, let afunction, $f:R - \left\{ { - a} \right\} \to R$ be defined by$f(x) = {{a - x} \over {a + x}}$. Further suppose that for any realnumber $x \ne - a$ and $f(x) \ne - a$, (fof)(x) = x. Then $f\left( { - {1 \over 2}} \right)$ is equal to :