This is very clever. Personally, I'd write it to call the functions from left to right, the way PIPE does. Normal people not accustomed to functional programming think of it as a quasi-imperative script. The only thing that bothers me about your approach is that it won't map a predicate over a stream so as to make a stream of Booleans. I don't often want to do that, but students do it all the time. (They should use KEEP instead, usually, but they don't.)
It's in MAP, not in KEEP, that we have to distinguish streams from regular lists. (Read my message again. ;~) )
What do you think about my example?
It should find the numbers inside the inner stream, rather than KEEPing the stream itself, which is what it does.
I'm not sure what you mean by "in front of" or "behind." So I don't really understand this whole section of your message.
This works unless the elements of the stream are scripts. I grant that that'd be unusual, but I wanted to make streams that could handle anything. So perhaps we could compromise by reusing item 2 for the evaluated tail, but keep the flag as item 3. But honestly I'd rather optimize time than space.