diff --git a/tests/regres/cov/coverage_test.go b/tests/regres/cov/coverage_test.go
index 6bd6b73..95aca12 100644
--- a/tests/regres/cov/coverage_test.go
+++ b/tests/regres/cov/coverage_test.go
@@ -222,16 +222,94 @@
 	//                         ╭┴╮      ╭╯
 	//                         v x      z
 	checkSpans(t, tree.Spans(), span0, span1, span2, span3)
+	checkGroups(t, tree.FileSpanGroups(fileA), map[cov.SpanGroupID]cov.SpanGroup{
+		0: cov.SpanGroup{Spans: spans(0, 1)},
+		1: cov.SpanGroup{Spans: spans(0, 3)},
+		2: cov.SpanGroup{Spans: spans(1, 2)},
+	})
 	checkTests(t, tree, `{a:{b:{d:{i} e:{j k}} c:{f:{l:{v x}} g:{n: {z} o}}}}`)
 	checkCoverage(t, tree, fileA, `a:{b:{d:{<0>} e:{[3]}} c:{f:{<2>} g:{n:{[2]} o:{<1>}}}}`)
 }
 
+func TestTreeOptInvertForCommon(t *testing.T) {
+	tree := &cov.Tree{}
+
+	tree.Add(cov.Path{"a", "b"}, coverage(fileA, span0))
+	tree.Add(cov.Path{"a", "c"}, coverage(fileA, span0))
+	tree.Add(cov.Path{"a", "d"}, coverage(fileA, span0))
+	tree.Add(cov.Path{"a", "e"}, coverage(fileA, span1))
+	tree.Add(cov.Path{"a", "f"}, coverage(fileA, span1))
+	tree.Add(cov.Path{"a", "g"}, coverage(fileA, span0))
+	tree.Add(cov.Path{"a", "h"}, coverage(fileA, span0))
+	tree.Add(cov.Path{"a", "i"}, coverage(fileA, span0))
+
+	//               (a)
+	//  ┏━━━┳━━━┳━━━┳━┻━┳━━━┳━━━┳━━━┓
+	// (b) (c) (d) (e) (f) (g) (h) (i)
+	// [0] [0] [0] [1] [1] [0] [0] [0]
+	checkSpans(t, tree.Spans(), span0, span1)
+	checkTests(t, tree, `{a:{b c d e f g h i}}`)
+	checkCoverage(t, tree, fileA, `a:{b:{[0]} c:{[0]} d:{[0]} e:{[1]} f:{[1]} g:{[0]} h:{[0]} i:{[0]}}`)
+
+	tree.Optimize()
+
+	//               [0]
+	//               (a)
+	//  ╭───┬───┬───┲━┻━┱───┬───┬───╮
+	//  b   c   d  ┏┛   ┗┓  g   h   i
+	//            (e)   (f)
+	//            <0>   <0>
+	checkSpans(t, tree.Spans(), span0, span1)
+	checkGroups(t, tree.FileSpanGroups(fileA), map[cov.SpanGroupID]cov.SpanGroup{
+		0: cov.SpanGroup{Spans: spans(0, 1)},
+	})
+	checkTests(t, tree, `{a:{b c d e f g h i}}`)
+	checkCoverage(t, tree, fileA, `a:{[0] e:{<0>} f:{<0>}}`)
+}
+
+func TestTreeOptDontInvertForCommon(t *testing.T) {
+	tree := &cov.Tree{}
+
+	tree.Add(cov.Path{"a", "b"}, coverage(fileA, span0))
+	tree.Add(cov.Path{"a", "c"}, coverage(fileA, span0))
+	tree.Add(cov.Path{"a", "d"}, coverage(fileA, span0))
+	tree.Add(cov.Path{"a", "e"}, coverage(fileA, span1))
+	tree.Add(cov.Path{"a", "f"}, coverage(fileA, span1))
+	tree.Add(cov.Path{"a", "g"}, coverage(fileA, span2))
+	tree.Add(cov.Path{"a", "h"}, coverage(fileA, span2))
+	tree.Add(cov.Path{"a", "i"}, coverage(fileA, span2))
+
+	//               (a)
+	//  ┏━━━┳━━━┳━━━┳━┻━┳━━━┳━━━┳━━━┓
+	// (b) (c) (d) (e) (f) (g) (h) (i)
+	// [0] [0] [0] [1] [1] [2] [2] [2]
+	checkSpans(t, tree.Spans(), span0, span1, span2)
+	checkTests(t, tree, `{a:{b c d e f g h i}}`)
+	checkCoverage(t, tree, fileA, `a:{b:{[0]} c:{[0]} d:{[0]} e:{[1]} f:{[1]} g:{[2]} h:{[2]} i:{[2]}}`)
+
+	tree.Optimize()
+
+	//               (a)
+	//  ┏━━━┳━━━┳━━━┳━┻━┳━━━┳━━━┳━━━┓
+	// (b) (c) (d) (e) (f) (g) (h) (i)
+	// [0] [0] [0] [1] [1] [2] [2] [2]
+	checkSpans(t, tree.Spans(), span0, span1, span2)
+	checkTests(t, tree, `{a:{b c d e f g h i}}`)
+	checkCoverage(t, tree, fileA, `a:{b:{[0]} c:{[0]} d:{[0]} e:{[1]} f:{[1]} g:{[2]} h:{[2]} i:{[2]}}`)
+}
+
 func checkSpans(t *testing.T, got []cov.Span, expect ...cov.Span) {
 	if !reflect.DeepEqual(got, expect) {
 		t.Errorf("Spans not as expected.\nGot:    %+v\nExpect: %+v", got, expect)
 	}
 }
 
+func checkGroups(t *testing.T, got, expect map[cov.SpanGroupID]cov.SpanGroup) {
+	if !reflect.DeepEqual(got, expect) {
+		t.Errorf("SpanGroupss not as expected.\nGot:    %+v\nExpect: %+v", got, expect)
+	}
+}
+
 func checkTests(t *testing.T, tree *cov.Tree, expect string) {
 	g, e := tree.Tests().String(tree.Strings()), expect
 	if tg, te := trimWS(g), trimWS(e); tg != te {
diff --git a/tests/regres/cov/optimization.go b/tests/regres/cov/optimization.go
index f2f357f..2d4ca8f 100644
--- a/tests/regres/cov/optimization.go
+++ b/tests/regres/cov/optimization.go
@@ -33,6 +33,9 @@
 		go func() {
 			defer wg.Done()
 			o := optimizer{}
+			for idx, tc := range file.tcm {
+				o.invertForCommon(tc, &t.testRoot.children[idx])
+			}
 			o.createGroups(file)
 		}()
 	}
@@ -41,6 +44,8 @@
 
 type optimizer struct{}
 
+// createGroups looks for common SpanSets, and creates indexable span groups
+// which are then used instead.
 func (o *optimizer) createGroups(f *treeFile) {
 	const minSpansInGroup = 2
 
@@ -71,7 +76,7 @@
 		spansets = append(spansets, &spansetInfo{
 			key: key,
 			set: set,
-			grp: SpanGroup{spans: set},
+			grp: SpanGroup{Spans: set},
 		})
 	}
 
@@ -100,8 +105,8 @@
 		for _, b := range spansets[i+1:] {
 			if len(a.set) > len(b.set) && a.set.containsAll(b.set) {
 				extend := b.id // Do not take address of iterator!
-				a.grp.spans = a.set.sub(b.set)
-				a.grp.extend = &extend
+				a.grp.Spans = a.set.removeAll(b.set)
+				a.grp.Extend = &extend
 				continue nextSpan
 			}
 		}
@@ -128,3 +133,39 @@
 		}
 	})
 }
+
+// invertCommon looks for tree nodes with the majority of the child nodes with
+// the same spans. This span is promoted up to the parent, and the children
+// have the span inverted.
+func (o *optimizer) invertForCommon(tc *TestCoverage, t *Test) {
+	wg := sync.WaitGroup{}
+	wg.Add(len(tc.Children))
+	for id, child := range tc.Children {
+		id, child := id, child
+		go func() {
+			defer wg.Done()
+			o.invertForCommon(child, &t.children[id])
+		}()
+	}
+	wg.Wait()
+
+	counts := map[SpanID]int{}
+	for _, child := range tc.Children {
+		for span := range child.Spans {
+			counts[span] = counts[span] + 1
+		}
+	}
+
+	for span, count := range counts {
+		if count > len(t.children)/2 {
+			tc.Spans = tc.Spans.invert(span)
+			for _, idx := range t.indices {
+				child := tc.Children.index(idx)
+				child.Spans = child.Spans.invert(span)
+				if child.deletable() {
+					delete(tc.Children, idx)
+				}
+			}
+		}
+	}
+}
diff --git a/tests/regres/cov/parser.go b/tests/regres/cov/parser.go
index 413d808..925e93c 100644
--- a/tests/regres/cov/parser.go
+++ b/tests/regres/cov/parser.go
@@ -176,16 +176,16 @@
 func (t *Tree) writeSpanGroupJSON(group SpanGroup, sb *strings.Builder) {
 	sb.WriteString(`{`)
 	sb.WriteString(`"s":[`)
-	for i, spanID := range group.spans.List() {
+	for i, spanID := range group.Spans.List() {
 		if i > 0 {
 			sb.WriteString(`,`)
 		}
 		sb.WriteString(fmt.Sprintf("%v", spanID))
 	}
 	sb.WriteString(`]`)
-	if group.extend != nil {
+	if group.Extend != nil {
 		sb.WriteString(`,"e":`)
-		sb.WriteString(fmt.Sprintf("%v", *group.extend))
+		sb.WriteString(fmt.Sprintf("%v", *group.Extend))
 	}
 	sb.WriteString(`}`)
 }
@@ -349,11 +349,11 @@
 			case "s":
 				p.array(func(spanIdx int) {
 					id := SpanID(p.integer())
-					g.spans[id] = struct{}{}
+					g.Spans[id] = struct{}{}
 				})
 			case "e":
 				extend := SpanGroupID(p.integer())
-				g.extend = &extend
+				g.Extend = &extend
 			}
 		})
 		spangroups[SpanGroupID(groupIdx)] = g
diff --git a/tests/regres/cov/tree.go b/tests/regres/cov/tree.go
index 7e6c3f2..2f54a82 100644
--- a/tests/regres/cov/tree.go
+++ b/tests/regres/cov/tree.go
@@ -76,14 +76,18 @@
 
 // Spans returns all the spans used by the tree
 func (t *Tree) Spans() SpanList {
-	out := make(SpanList, 0, len(t.spans))
-	for span := range t.spans {
-		out = append(out, span)
+	out := make(SpanList, len(t.spans))
+	for span, id := range t.spans {
+		out[id] = span
 	}
-	sort.Slice(out, func(i, j int) bool { return out[i].Before(out[j]) })
 	return out
 }
 
+// FileSpanGroups returns all the span groups for the given file
+func (t *Tree) FileSpanGroups(path string) map[SpanGroupID]SpanGroup {
+	return t.files[path].spangroups
+}
+
 // FileCoverage returns the TestCoverageMap for the given file
 func (t *Tree) FileCoverage(path string) TestCoverageMap {
 	return t.files[path].tcm
@@ -143,7 +147,7 @@
 		for _, indexedTest := range tests {
 			if indexedTest.created {
 				if parent != nil && len(test.children) == 1 {
-					parent.Spans = parent.Spans.add(spans)
+					parent.Spans = parent.Spans.addAll(spans)
 					delete(parent.Children, indexedTest.index)
 				} else {
 					tc := tcm.index(indexedTest.index)
@@ -157,19 +161,19 @@
 
 			// If the tree node contains spans that are not in this new test,
 			// we need to push those spans down to all the other children.
-			if lower := tc.Spans.sub(spans); len(lower) > 0 {
+			if lower := tc.Spans.removeAll(spans); len(lower) > 0 {
 				// push into each child node
 				for i := range test.children {
 					child := tc.Children.index(TestIndex(i))
-					child.Spans = child.Spans.add(lower)
+					child.Spans = child.Spans.addAll(lower)
 				}
 				// remove from node
-				tc.Spans = tc.Spans.sub(lower)
+				tc.Spans = tc.Spans.removeAll(lower)
 			}
 
 			// The spans that are in the new test, but are not part of the tree
 			// node carry propagating down.
-			spans = spans.sub(tc.Spans)
+			spans = spans.removeAll(tc.Spans)
 			if len(spans) == 0 {
 				continue nextFile
 			}
@@ -288,9 +292,15 @@
 	return sb.String()
 }
 
+// deletable returns true if the TestCoverage provides no data.
+func (tc TestCoverage) deletable() bool {
+	return len(tc.Spans) == 0 && tc.Group == nil && len(tc.Children) == 0
+}
+
 // TestCoverageMap is a map of TestIndex to *TestCoverage.
 type TestCoverageMap map[TestIndex]*TestCoverage
 
+// traverse performs a depth first traversal of the TestCoverage tree.
 func (tcm TestCoverageMap) traverse(cb func(*TestCoverage)) {
 	for _, tc := range tcm {
 		cb(tc)
@@ -380,16 +390,31 @@
 	return sb.String()
 }
 
+func (s SpanSet) contains(rhs SpanID) bool {
+	_, found := s[rhs]
+	return found
+}
+
 func (s SpanSet) containsAll(rhs SpanSet) bool {
 	for span := range rhs {
-		if _, found := s[span]; !found {
+		if !s.contains(span) {
 			return false
 		}
 	}
 	return true
 }
 
-func (s SpanSet) sub(rhs SpanSet) SpanSet {
+func (s SpanSet) remove(rhs SpanID) SpanSet {
+	out := make(SpanSet, len(s))
+	for span := range s {
+		if span != rhs {
+			out[span] = struct{}{}
+		}
+	}
+	return out
+}
+
+func (s SpanSet) removeAll(rhs SpanSet) SpanSet {
 	out := make(SpanSet, len(s))
 	for span := range s {
 		if _, found := rhs[span]; !found {
@@ -399,7 +424,16 @@
 	return out
 }
 
-func (s SpanSet) add(rhs SpanSet) SpanSet {
+func (s SpanSet) add(rhs SpanID) SpanSet {
+	out := make(SpanSet, len(s)+1)
+	for span := range s {
+		out[span] = struct{}{}
+	}
+	out[rhs] = struct{}{}
+	return out
+}
+
+func (s SpanSet) addAll(rhs SpanSet) SpanSet {
 	out := make(SpanSet, len(s)+len(rhs))
 	for span := range s {
 		out[span] = struct{}{}
@@ -410,18 +444,25 @@
 	return out
 }
 
+func (s SpanSet) invert(rhs SpanID) SpanSet {
+	if s.contains(rhs) {
+		return s.remove(rhs)
+	}
+	return s.add(rhs)
+}
+
 // SpanGroupID is an identifier of a SpanGroup.
 type SpanGroupID int
 
 // SpanGroup holds a number of spans, potentially extending from another
 // SpanGroup.
 type SpanGroup struct {
-	spans  SpanSet
-	extend *SpanGroupID
+	Spans  SpanSet
+	Extend *SpanGroupID
 }
 
 func newSpanGroup() SpanGroup {
-	return SpanGroup{spans: SpanSet{}}
+	return SpanGroup{Spans: SpanSet{}}
 }
 
 func indent(s string) string {