Wang tiles have also been used in cellular automata theory decidability proofs. Completeness, consistency, decidability, and expressivity, are further fundamental concepts in logic. Methods used to establish decidability include quantifier elimination, model completeness, and Vaught's test. He was a mathematician noted for his work on the decidability of various algebraic groups. Remarkably, decidability of admissibility in the basic modal logic "K" is a major open problem. The concept of decidability may be extended to other models of computation. The basic decidability results for admissible rules also carry to rules with parameters. There are several basic results about decidability of theories. It is a conditional statement about the decidability of positivity of a real number. A property of a theory or logical system weaker than decidability is semidecidability. The question of decidability is still unresolved. There are many more similar problems whose decidability can be proved in a similar way. Decidability should not be confused with completeness.