unique objects

[Chris Muller]

> This is not correct: MagmaSet>>#add: will signal a
> MagmaDuplicateObjectInCollection notification when an attempt to add
> duplicate object.

Ok, yes.  But do you know why we made it inconsistent from a standard Set?

|o|
o := Object new.
Set new
	add: o; add: o; size  "1"

Cheers,
  Chris