How to handle non-boolean variables

[GiggleLiu]

Some random thoughts.

For boolean variables, a literal can be positive, negative or absent.
For 3-state variables, a literal can be (1), (2), (3), (1, 2), (2, 3), (1, 3) or absent.

It grows exponentially as $2^k - 1$ for a k-state variable. We may consider implementing it for handling 3-coloring or integer programming problems.