func BenchmarkLBusy(b *testing.B) {
	r := singletasking.LRunner{}
	r.Run(blocker)
	for i := 0; i < b.N; i++ {
		r.Run(blocker)
	}
}
func BenchmarkLUnblocked(b *testing.B) {
	r := singletasking.LRunner{}
	for i := 0; i < b.N; i++ {
		r.Run(void)
	}
}