How many questions do you need to prove that unasked questions have no answers?

Suppose a quantum system is prepared in an arbitrary quantum state. How many yes-no questions about that system would you have to consider to prove that such questions have no predefined answers? Peres conjectured that the minimum number was 18, as in the case of the set found in 1995. Asher's conjecture has recently been proven correct: there are no sets with fewer than 18 questions. This is the end of a long story which began in 1967, when Kochen and Specker found a similar set requiring 117 questions.