If you look at this (or attached, if it doesn't show up inline) picture  from Eliot Miranda's  Closure's Part I discussion ,  the naive <cough> me </cough> would spend two days wondering whey there are no senders.
After all, its obvious that  --sender: nil means that there are no senders.

From Squeak By Example, page 55 in the discussion of pseudo-variables, "nil is the unique instance of the class UndefinedObject".

Which makes perfect sense.

One of these days, I will get past paragraph 4 on this Closures Discussion, but boy, there are a lot of things going on under the hood there. (:

Thanks for your patience as I try to grok this stuff.