= Solution
The required <function composition> is
``
unfold p f g = map f . takeWhile (not . p) . iterate g
``
`iterate g` supplies the successive states, `takeWhile (not . p)` retains precisely those before the first stopping state, and `map f` produces their outputs. If the initial state satisfies the stopping predicate, both sides return the empty list. Otherwise both emit the same head and repeat from `g x`; the same argument applies to every demanded finite prefix of an infinite result.
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