add test condtions TestPollardNilSwap

it does not trigger the problem in issue #337 though.  It just swaps 1, 2

accumulator/pollard.go

@@ -394,6 +394,8 @@ func (p *Pollard) swapNodes(s arrow, row uint8) (*hashableNode, error) {
 	if bhn.sib.niece[0].data == empty || bhn.sib.niece[1].data == empty {
 		bhn = nil // we can't perform this hash as we don't know the children
 	}
+
+	fmt.Printf("swap %d %d OK\n", s.from, s.to)
 	return bhn, nil
 }
 

accumulator/pollard_test.go

@@ -56,6 +56,36 @@ func TestPollardSimpleIngest(t *testing.T) {
 	}
 }
 
+func TestPollardNilSwap(t *testing.T) {
+	var p Pollard
+	adds := make([]Leaf, 8)
+	for i := 0; i < len(adds); i++ {
+		adds[i].Hash[0] = uint8(i)
+		adds[i].Hash[20] = 0xff
+		adds[i].Remember = true
+	}
+
+	adds[6].Remember = false
+	adds[7].Remember = false
+
+	p.Modify(adds, nil)
+	fmt.Println(p.ToString())
+
+	adds[6].Remember = true
+	adds[7].Remember = true
+
+	p.Modify(adds, nil)
+	fmt.Println(p.ToString())
+
+	dels := []uint64{8, 9, 10, 11, 12, 13, 14, 15}
+	p.Modify(nil, dels)
+	fmt.Println(p.ToString())
+
+	dels = []uint64{1, 3, 4, 5}
+	p.Modify(nil, dels)
+	fmt.Println(p.ToString())
+}
+
 func pollardRandomRemember(blocks int32) error {
 	f := NewForest(RamForest, nil, "", 0)