Apologies if this isn't the best place to post corrections for the Pharo book. I'm using the latest PBE I can find (PBE1-2010-02-01.pdf). On page 200 I think all references to #xor: should be replaced by #bitXor: �as my pharo-1.0 image doesn't contain #xor: except on False>>xor: and True>>xor: So Book>>hash should read: