for yield(f0) {
		f0, f1 = f1, f0+f1
	}
}

// print the Fibonacci numbers below 1000:
for x := range fibo {
	if x >= 1000 {
		break
	}
	fmt.Printf("%d ", x)
}
// output: 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

// iteration support for a recursive tree data structure
type Tree[K cmp.Ordered, V any] struct {
	left, right *Tree[K, V]
	key         K
	value       V