Levente Uzonyi wrote:
> Sets are not meant to be used for error detection.

Precisely. The issue is about consistency, not error handling.
Take this, for example:

   Array with: nil.
   OrderedCollection with: nil.
   Bag with: nil.
   Set with: nil.

If the first three work, why wouldn't the last? Or this one:

   (OrderedCollection new) add: nil.
   (Bag new) add: nil.
   (Dictionary new) at: nil put: nil.
   (Set new) add: nil.

If the first three work, why doesn't the last one? All of the above 
claim to be general purpose collections (i.e., contrary to collections 
that only store certain kinds of objects such as ByteArray or String). 
In this context it seems just silly that Set wouldn't store nil given 
that Array, OrderedCollection, Bag, Dictionary can store nil just fine.

Cheers,
   - Andreas