  We give an explicit characterization of all injective subsets of the model
space $l_{\infty}(I)$ for a general set $I$, in terms of inequalities involving
$1$-Lipschitz functions. Since the class of all injective metric spaces
coincides with the one of all absolute $1$-Lipschitz retracts, the present work
yields a characterization of all the subsets of $l_{\infty}(I)$ that are
absolute $1$-Lipschitz retracts.
