It would have been easy enough to conclude that this whole castle was, 
like the others, a Turing machine. But the Primer had taught Nell to be very 
careful about making unwarranted assumptions. Just because her stall functioned 
according to Turing rules did not mean that all of the others did. And even if 
every stall in this castle was, in fact, a Turing machine, she still could not 
come to any fixed conclusions. She had seen riders carrying books to and from 
the castle, which meant that cipherers must be at work elsewhere in this 
kingdom. She could not verify that all of them were Turing machines.