Suppose someone got stuck solving a Sudoku and asks you to figure out, what went wrong. Unfortunately that person only sends you a copy of the instance, where you neither see which of the numbers constituted to the initial set of numbers, nor do you get any information about the order in which the other numbers have been entered.  
Note that some of the fields may not have entries because the Sudoku was not finished when the contradiction to the filling rules was realized.

>**Question:**   
what is the complexity of detecting the minimal set of entries, whose removal would restore the solvability of the Sudoku instance?