Tim Budd has already been mentioned, but I would like to reinforce that
by suggesting that anyone interested in this subject read chapter 8 of
"A Little Smalltalk":

http://stephane.ducasse.free.fr/FreeBooks/LittleSmalltalk/ALittleSmallta
lk.pdf

The implementation of generators in Smalltalk is shown (Little Smalltalk
uses that instead of Streams) and several interesting examples are
given.

-- Jecel