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this won't work. because, in (let ((a <e1>) (b <e2>))), when compute e2, it depends y, but we only have a not y. For the same reason, the solution in text will work.
I think both method will work, because (eval "dy") is 'delayed'.
But the expression "(stream-map f y)" defined in dy will be evaluated while y hasn't been defined yet.
So former method won't work.