Returns a tentative variable reflecting the (current and future) violatedness of the constraint set.
    ?- lib(tentative_constraints).	% for alldifferent/1
    Yes (0.00s cpu)
    ?- length(Xs, 5), tent_set_all(Xs, 99), CS :~ alldifferent(Xs),
       cs_violations(CS, V).
    Xs = [Xi{99 -> 4}, Xi{99 -> 4}, Xi{99 -> 4}, Xi{99 -> 4}, Xi{99 -> 4}]
    CS = constraint_set(V{10 -> 0}, ...)
    V = V{10 -> 0}
    There is 1 delayed goal.
    Yes (0.00s cpu)