> 
> Stéphane, can you live with ifMissingAdd: ?  Chris?
> 

Sure. Now "missing" seems to imply that something went wrong and an 
element is not there while it should (which is why I would prefer 
#ifAbsentAdd:).

Plus, the name does not tell that a Boolean is returned (this apply to 
#ifAbsentAdd: too).

So what about #ensurePresenceOf: ?

I would expect this to return true if the element was *not* added, false 
otherwise, though.


Stef