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

pkg go/doc, type Example struct, Comments []*ast.CommentGroup
pkg go/doc, type Example struct, Doc string
pkg go/doc, type Example struct, Name string
pkg go/doc, type Example struct, Output string
pkg go/doc, type Filter func(string) bool
pkg go/doc, type Func struct
pkg go/doc, type Func struct, Decl *ast.FuncDecl
pkg go/doc, type Func struct, Doc string
pkg go/doc, type Func struct, Level int