I am not good with and I am very interested in combinational logic. I get lost in these variables: there are four of them and each can be in many states: like 'absent', 'present', 'wrong format', 'bad word' etc. Even if each of them has 4 states then I have 2^4=16 combinations. I don't think it's a good idea to provide 16 tests for each combination but I can't see how to make the number smaller. :(