  A simple model of a frustrated disordered system is presented. Apart from the
(very different) physical interpretation, the model shares many features with
that of Sherrington-Kirkpatrick for spin glasses, but, as a consequence of its
relative simplicity, its ground state can be exactly determined by numerical
methods. This fact allows us to test experimentally some theoretical
predictions, based on a specialization of the ``cavity method'' developed for
the SK model, which is presently limited to a ``non-frustrated'' approximation,
corresponding to some extent to the replica-symmetric one for the SK model.
