At page 202 in the book Pharo by Exmaple, it says: "A difficult error to spot is when you redefine = but not hash. The symptoms are that you will lose elements that you put in sets or other strange behaviour. ..." I have come up of an idea that can blow this away. HashProof is a trait that guarantees that #hash and #= are defined correctly. If you file in this http://forum.world.st/file/n3809543/HashProof.st HashProof.st , You will see HashProofTrait computes #= and #hash by using an array answered by the trait user by sending message #gettersForEquality. HashProofUser uses HashProofTrait. Try example below, u := HashProofUser new. u x:1. u y:2. u z:3. v := HashProofUser new. v x:1. v y:2. v z:3. u = v. User of the HashProofTrait is meets an error undefined message #gettersForEquality, when it was first sent #= or #hash. Therefore, it is very clear what he has to do to make #= and #hash work flawlessly. HwaJong Oh -- View this message in context: http://forum.world.st/HashProof-tp3809543p3809543.html Sent from the Pharo Smalltalk mailing list archive at Nabble.com.