Actually, my first idea was to change Run_aut so that the fact that
several programs were run in parallel was hidden. But it turned out
to be a bad idea, because of numeric unsatisfiability + the
interaction between the module that choses a formula, and the one
that draw (performing the numeric satisfiability solving). Indeed,
when a formula appear to be unsatisfiable for numeric reason, we need
to backtrack in via Run_aut.choose_on_formula in one of the
Run_aut.t ; but of course if the list of Run_aut.t is hidden, we are
dead. One solution would have been to complicate a little bit the
Run_aut.t interface ; but my first motivation was actually to
simplify it...