Conditional construct for a kleenean data type

Question

I was thinking of an hypothetical programming language with a kleenean data type which would implement Kleene's three-valued logic. To sum up, it's an extension of the boolean data type with the three constants true, false and unknown where unknown means that the value is either true or false, but we don't know which.

The truth tables for a kleenean type are well-known and the logic is quite easy to understand. However, I was wondering how one would design a conditional construct to take in account this unknown value.

A basic if-then-else conditional construct is almost always as follows:

if (boolean condition) then
    condition is true, do something
else
    condition is false, do some other thing
end

However, I have troubles seeing what a kleenean if construct would look like. How could we interpret the unknown constant? Technically speaking, it could satisfy the true condition as well as the false condition since it is one of these two. However, we can't have it match any of those since it could be the other, it is not really true nor false.

Is there a well-known way to implement such a construct?

EDIT: To specify a little bit, I would prefere something different than the way boost::tribool works, or from a simple switch as if was an enum. Answers about quantum superposition and semantics are welcome.