diff --git a/debug.go b/debug.go
index bc46463..2a397fa 100644
--- a/debug.go
+++ b/debug.go
@@ -76,15 +76,20 @@ func (n *node) fprintBST(w io.Writer, pad string) error {
 	return nil
 }
 
-// Statistics, returns the maxDepth, average and standard deviation of the nodes.
+// statistics, returns the maxDepth, average and standard deviation of the nodes.
+// If the skip function is not nil, a true return value defines which nodes must be skipped in the statistics.
 //
 // Note: This is for debugging and testing purposes only during development.
-func (t Table) statistics() (size int, maxDepth int, average, deviation float64) {
+func (t Table) statistics(skip func(netip.Prefix, any, int) bool) (size int, maxDepth int, average, deviation float64) {
 	// key is depth, value is the sum of nodes with this depth
 	depths := make(map[int]int)
 
 	// closure callback, get the depths, sum up the size
 	cb := func(pfx netip.Prefix, val any, depth int) bool {
+		if skip != nil && skip(pfx, val, depth) {
+			return true
+		}
+
 		depths[depth] += 1
 		size += 1
 		return true
diff --git a/treap_test.go b/treap_test.go
index 93ae4b0..b62fc99 100644
--- a/treap_test.go
+++ b/treap_test.go
@@ -121,6 +121,10 @@ func TestZeroValue(t *testing.T) {
 	if _, _, ok := zeroTable.LookupCIDR(zeroCIDR); ok {
 		t.Errorf("LookupCIDR(), got: %v, want: false", ok)
 	}
+
+	if rtbl := zeroTable.Union(&zeroTable); *rtbl != zeroTable {
+		t.Errorf("Union(), got: %#v, want: %#v", rtbl, &zeroTable)
+	}
 }
 
 func TestInsert(t *testing.T) {
@@ -534,6 +538,12 @@ func TestUnion(t *testing.T) {
 		t.Errorf("Fprint()\nwant:\n%sgot:\n%s", asTopoStr, rtbl.String())
 	}
 
+	clone := rtbl.Clone()
+	rtbl.UnionMutable(new(cidrtree.Table))
+	if !reflect.DeepEqual(rtbl, clone) {
+		t.Fatal("UnionMutable with zero value changed original")
+	}
+
 	rtbl3 := rtbl.Union(rtbl2)
 	if rtbl3.String() != asTopoStr {
 		t.Errorf("Fprint()\nwant:\n%sgot:\n%s", asTopoStr, rtbl.String())
diff --git a/whitebox_test.go b/whitebox_test.go
index 87f15df..786cb5a 100644
--- a/whitebox_test.go
+++ b/whitebox_test.go
@@ -18,7 +18,7 @@ func TestFprintBST(t *testing.T) {
 		rtbl.InsertMutable(randPfx4(), nil)
 		rtbl.InsertMutable(randPfx6(), nil)
 	}
-	size, maxDepth, average, deviation := rtbl.statistics()
+	size, maxDepth, average, deviation := rtbl.statistics(nil)
 	t.Logf("v4/v6: size: %10d, maxDepth: %4d, average: %3.2f, deviation: %3.2f", size, maxDepth, average, deviation)
 
 	t.Log()
@@ -37,7 +37,7 @@ func TestStatisticsRandom(t *testing.T) {
 		for c := 0; c <= i; c++ {
 			rtbl.InsertMutable(randPfx(), nil)
 		}
-		size, maxDepth, average, deviation := rtbl.statistics()
+		size, maxDepth, average, deviation := rtbl.statistics(nil)
 		t.Logf("v4/v6: size: %10d, maxDepth: %4d, average: %3.2f, deviation: %3.2f", size, maxDepth, average, deviation)
 	}
 }
@@ -47,8 +47,29 @@ func TestStatisticsFullTable(t *testing.T) {
 	for _, cidr := range fullTable {
 		rtbl.InsertMutable(cidr, nil)
 	}
-	size, maxDepth, average, deviation := rtbl.statistics()
-	t.Logf("FullTable: size: %10d, maxDepth: %4d, average: %3.2f, deviation: %3.2f", size, maxDepth, average, deviation)
+
+	skip4 := func(pfx netip.Prefix, val any, depth int) bool {
+		if pfx.Addr().Is4() {
+			return true
+		}
+		return false
+	}
+
+	skip6 := func(pfx netip.Prefix, val any, depth int) bool {
+		if !pfx.Addr().Is4() {
+			return true
+		}
+		return false
+	}
+
+	size, maxDepth, average, deviation := rtbl.statistics(skip6)
+	t.Logf("FullTableV4: size: %10d, maxDepth: %4d, average: %3.2f, deviation: %3.2f", size, maxDepth, average, deviation)
+
+	size, maxDepth, average, deviation = rtbl.statistics(skip4)
+	t.Logf("FullTableV6: size: %10d, maxDepth: %4d, average: %3.2f, deviation: %3.2f", size, maxDepth, average, deviation)
+
+	size, maxDepth, average, deviation = rtbl.statistics(nil)
+	t.Logf("FullTable:   size: %10d, maxDepth: %4d, average: %3.2f, deviation: %3.2f", size, maxDepth, average, deviation)
 }
 
 func TestLPMRandom(t *testing.T) {
@@ -57,7 +78,7 @@ func TestLPMRandom(t *testing.T) {
 		for c := 0; c <= i; c++ {
 			rtbl.InsertMutable(randPfx(), nil)
 		}
-		size, maxDepth, average, _ := rtbl.statistics()
+		size, maxDepth, average, _ := rtbl.statistics(nil)
 		var lpm netip.Prefix
 		var depth int
 
@@ -82,7 +103,7 @@ func TestLPMFullTableWithDefaultRoutes(t *testing.T) {
 	rtbl.InsertMutable(dg4, nil)
 	rtbl.InsertMutable(dg6, nil)
 
-	size, maxDepth, average, deviation := rtbl.statistics()
+	size, maxDepth, average, deviation := rtbl.statistics(nil)
 	t.Logf("FullTable: size: %10d, maxDepth: %4d, average: %3.2f, deviation: %3.2f", size, maxDepth, average, deviation)
 	t.Log()