Calling var_inc_violations/2 on a nonvariable has no effect and silently succeeds.
    ?- tent_set(X, 27), var_inc_violations(X, 3).
    X = X{27 -> 3}
    Yes (0.00s cpu)
    ?- tent_set(X, 27), var_inc_violations(X, 3), var_inc_violations(X, 1).
    X = X{27 -> 4}
    Yes (0.00s cpu)
    ?- tent_set(X, 27), var_inc_violations(X, 3), var_inc_violations(X, -1).
    X = X{27 -> 2}
    Yes (0.00s cpu)
    ?- var_inc_violations(foo, 3).
    Yes (0.00s cpu)
    ?- var_inc_violations(_, 3).
    No (0.00s cpu)