I can't really tell what are you asking about, but to override hash, you should do the same as with =
, which you have overridden as well, just by including different definition. So you do something like:
hash [
"return hash here"
]
If you are asking what hash should be like… well think about it like that: objects that are equal have to have the same hash (but this doesn't have to work the other way around). So I'd suggest you to do something like
hash [
^ self getIsbn hash
]
Also about disappearing element. Set is a hashed collection. This means that before comparing it's element with the one you are looking for, it select's a subset by hash. So if you are not overriding hash it may select a subset that won't contain your desired element.
In the end I'd suggest you to use some different implementation of Smalltalk, because my head hurt when I was starting to learn it with gnu-smalltalk. Personaly I use pharo it provides a nice UI and allows you to see what you override, allows you to debug, etc.