panic("failed to stabilize NumGoroutine after 1000 iterations")
}

func TestPullDoubleNext(t *testing.T) {
	next, _ := Pull(doDoubleNext())
	nextSlot = next
	next()
	if nextSlot != nil {
		t.Fatal("double next did not fail")
	}
}

var nextSlot func() (int, bool)

func doDoubleNext() Seq[int] {
	return func(_ func(int) bool) {
		defer func() {
			if recover() != nil {
				nextSlot = nil
			}