Indeed, it looks like SOME is not supported in Isabelle/ZF. You may want to make sure by asking on the Isabelle mailing list.
I have done quite a lot of proofs in Isabelle/ZF and I have never needed SOME. Can provide a small example of how you would use SOME if it worked?
Maybe I will come up with some other approach that is sufficient for what you want to do.
THE is only usable when you have uniqueness.
Slawekk
--- On Fri, 3/6/09, Victor Porton <por...@narod.ru> wrote: