When a variable becomes untenable, it appears in the set of conflict variable, when it becomes tenable, is disappears. This primitive returns the list of all currently untenable variables. Note that all these variables must be reassigned in any solution (there is no other way to repair untenability). Variable reassignment can be achieved by changing the variable's tentative value with tent_set/2, or by instantiating the variable. Care should be taken whilst implementing repairs through tentative value changes since this is a non-monotonic operation: conflicting repairs may lead to cycles and the computation may not terminate.