Adds a given object to the set, if it is not already a member.
Original signature is '- (void)addObject:(id)object'
Available in Mac OS X v10.0 and later.


- object (Id)
- The object to add to the set.

- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later), 10.5 (For Mac OS X 10.5 and later)