20.1.2

题目

Define a stream that yields successive elements of the Fibonacci sequence $(1, 1, 2, 3, 5, 8, 13, \dots)$ .

\begin{array}{r} fib = fix (λ f : Nat \to Nat \to Stream . λ n : Nat . λ m : Nat . λ_: Unit . \\ {n, f m (plus n m))}) 1 1 \end{array}