Line 1, is quite a dangerous assignment, as it “quietly” removes Xs and Zs in the original 4 state variable
without any warning. So $isunknown() check has to be used before this assignment.
The only question what would you do if $isunknown() returns true… Assigning da to 0 is one solution,
but that is equivalent to assigning a bit vector of 48 zeros to da. I would even suggest a $fatal() if
Xs and Zs are found.