>From: Roel Wuyts <wuyts at iam.unibe.ch>
>... this sounds a lot like the nested lazy streams I use in the previous
>implementation of SOUL. These streams are created as wrappers around
>existing collections or use a general block to define whatever you want
>(such as infinite streams of numbers etc.). 

The way I do this is to define select: and collect: on streams.
These methods return a SelectStream and a CollectStream, which
are probably the same thing as your nested lazy streams.  This
way you can treat streams about the same as collections.  My rule
of thumb is that I use collections for reasonably small groups,
and streams for large or infinite groups.  It would be reasonable
to define to:do: on streams, but I haven't needed it yet.

-Ralph