Out contracts: how to refer to objects' start state

[Peter Williams]

For some class methods, to express comprehensive out{} contracts it is 
necessary to be able to refer to the state of the class object before 
the operation as well as after it e.g. if the method adds something to a 
container you need to be able to specify that nothing was accidentally 
deleted from the container during the method.  I've scoured the language 
specification and Andrei's book for advice on how to do this without any 
luck.

Can it be done and, if so, how?

Thanks
Peter