Tentative value implementation of disequality constraint.
The violatedness of the constraint is 0 if the disequality holds, 1 otherwise.
The following declaration is in effect, meaning that neq_t/3 is used whenever $\= /2 is added to a constraint set:
:- neq_t/3 tent_implements $\= /2.
    ?- [X, Y] tent_set [3, 4], CS :~ (X $\= Y).
    X = X{3 -> 0}
    Y = Y{4 -> 0}
    CS = constraint_set(TotalVio{0 -> 0}, ...)
    There is 1 delayed goal.
    Yes (0.00s cpu)
    ?- [X, Y] tent_set [3, 3], CS :~ (X $\= Y).
    X = X{3 -> 0}
    Y = Y{3 -> 0}
    CS = constraint_set(TotalVio{1 -> 0}, ...)
    There is 1 delayed goal.
    Yes (0.00s cpu)