X δbtail: tests: Clone after second update

        $ time go test -failfast -short -run 'ΔBTail/rebuild' >x

before:

        real    1m15,669s
        user    1m31,340s
        sys     0m10,171s

after:

	real    0m43,071s
	user    0m57,487s
	sys     0m4,869s

wcfs/δbtail_test.go

@@ -825,6 +825,7 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 // assertTrack verifies that trackIdx == trackIdxOK.
 // XXX place
 func assertTrack(t *testing.T, subj string, trackIdx, trackIdxOK trackIndex) {
+	t.Helper()
 	if !reflect.DeepEqual(trackIdx, trackIdxOK) {
 		t.Errorf("%s:\n\thave: %v\n\twant: %v", subj, trackIdx, trackIdxOK)
 	}
@@ -833,6 +834,7 @@ func assertTrack(t *testing.T, subj string, trackIdx, trackIdxOK trackIndex) {
 // assertTrack verifies state of .trackIdx and .trackNew.
 // XXX place
 func (δbtail *ΔBtail) assertTrack(t *testing.T, subj string, trackIdxOK, trackNewOK trackIndex) {
+	t.Helper()
 	assertTrack(t, subj + ": trackIdx", δbtail.trackIdx, trackIdxOK)
 	assertTrack(t, subj + ": trackNew", δbtail.trackNew, trackNewOK)
 
@@ -883,72 +885,57 @@ func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1
 			t.Run(fmt.Sprintf(" T%s;R", keys1), func(t *testing.T) {
 				δbtail := NewΔBtail(t0.at, db)
 
-				// FIXME do Update(t0->t1) only once + clone after it
-
 				// assert trackIdx=ø, trackNew=ø, vδB=[]
 				δbtail.assertTrack(t, "@at0", ø, ø)  // XXX + vδB
-				step1 := func() { // XXX will be not needed with clone
-				xverifyΔBTail_rebuild1(t, db, δbtail, t0, t1, treeRoot, xat,
-					// after Update(t0->t1):
-					/*trackIdx=*/ ø,
 
+				xverifyΔBTail_rebuild_U(t, δbtail, t0, t1, xat, /*trackIdx=*/ø)
+				xverifyΔBTail_rebuild_TR(t, db, δbtail, t1, treeRoot, xat,
 					// after Track(keys1)
 					keys1,
+					/*trackIdx=*/ ø,
 					/*trackNew=*/ t1.xkv.trackIdx(keys1),
 
 					// after rebuild
-					/*trackIdx=*/    t1.xkv.trackIdx(keys1),
+					/*trackIdx=*/ t1.xkv.trackIdx(keys1),
 					/*vδB=*/      /*XXX temp*/nil /*[δ1/Tadj(keys1)*/)
-				}
-				step1()
 
+				t.Run(fmt.Sprintf(" →%s", t2.tree), func(t *testing.T) {
+					// tracked keys1 becomes tracked keys1_2 after Update(t1->t2)
+					keys1_2 := kadj12.Map(keys1)
+					xverifyΔBTail_rebuild_U(t, δbtail, t1, t2, xat, /*trackIdx=*/t2.xkv.trackIdx(keys1_2))
+
+					// tRestKeys2 = tAllKeys - keys1
+					tRestKeys2 := tAllKeys.Difference(keys1)
+					tRestKeyv2 := tRestKeys2.SortedElements()
+					for k2idx := range IntSets(len(tRestKeyv2)) {
+						keys2 := SetKey{}
+						for _, idx2 := range k2idx {
+							keys2.Add(tRestKeyv2[idx2])
+						}
 
-				// tRestKeys2 = tAllKeys - keys1
-				tRestKeys2 := tAllKeys.Difference(keys1)
-				tRestKeyv2 := tRestKeys2.SortedElements()
-				for k2idx := range IntSets(len(tRestKeyv2)) {
-					keys2 := SetKey{}
-					for _, idx2 := range k2idx {
-						keys2.Add(tRestKeyv2[idx2])
+						t.Run(fmt.Sprintf(" T%s;R", keys2), func(t *testing.T) {
+							δbtail_ := δbtail.clone()
+							xverifyΔBTail_rebuild_TR(t, db, δbtail_, t2, treeRoot, xat,
+								// after Track(keys2)
+								keys2,
+								// FIXME vvv trackNew should not cover ranges that are already in trackIdx
+								/*trackIdx*/ t2.xkv.trackIdx(keys1_2),
+								/*trackNew*/ t2.xkv.trackIdx(keys2),
+
+								// after rebuild
+								/* trackIdx=*/ t2.xkv.trackIdx(keys1_2.Union(keys2)),
+								/*vδB=*/      /*XXX temp*/nil /*[δ1/keys1+keys2, δ2/keys1+keys2]*/)
+						})
 					}
-
-					t.Run(fmt.Sprintf(" →%s T%s;R", t2.tree, keys2), func(t *testing.T) {
-
-						//δbtail = NewΔBtail(t0.at, db)
-						//step1()
-						//δbtail_ := δbtail
-						δbtail_ := δbtail.clone()
-
-						// FIXME do Update(t1->t2) only once + clone after it
-
-
-						// tracked keys1 becomes tracked keys1_2 after Update(t1->t2)
-						keys1_2 := kadj12.Map(keys1)
-
-
-						xverifyΔBTail_rebuild1(t, db, δbtail_, t1, t2, treeRoot, xat,
-							// after Update(t1->t2):
-							/*trackIdx=*/ t2.xkv.trackIdx(keys1_2),
-
-							// after Track(keys2)
-							keys2,
-							// FIXME vvv trackNew should not cover ranges that are already in trackIdx
-							/*trackNew*/ t2.xkv.trackIdx(keys2),
-
-							// after rebuild
-							/* trackIdx=*/ t2.xkv.trackIdx(keys1_2.Union(keys2)),
-							/*vδB=*/      /*XXX temp*/nil /*[δ1/keys1+keys2, δ2/keys1+keys2]*/)
-					})
-				}
+				})
 			})
 		}
 	})
 }
 
-// xverifyΔBTail_rebuild1 verifies ΔBTree state:
-// 1. after Update(ti->tj),
-// 2. after further Track(keys), rebuild
-func xverifyΔBTail_rebuild1(t *testing.T, db *zodb.DB, δbtail *ΔBtail, ti, tj *tTreeCommit, treeRoot zodb.Oid, xat map[zodb.Tid]string, trackIdx trackIndex, keys SetKey, trackNew, trackIdxAfterRebuild trackIndex,vδBok []ΔB) {
+// xverifyΔBTail_rebuild_U verifies ΔBTree state after Update(ti->tj).
+func xverifyΔBTail_rebuild_U(t *testing.T, δbtail *ΔBtail, ti, tj *tTreeCommit, xat map[zodb.Tid]string, trackIdx trackIndex) {
+	t.Helper()
 	X := exc.Raiseif
 	ø := trackIndex{}
 
@@ -958,6 +945,13 @@ func xverifyΔBTail_rebuild1(t *testing.T, db *zodb.DB, δbtail *ΔBtail, ti, tj
 	δbtail.assertTrack(t, fmt.Sprintf("after Update(@%s→@%s)", xat[ti.at], xat[tj.at]), trackIdx, ø)
 	// XXX vδB=[ø]
 	_ = δB
+}
+
+// xverifyΔBTail_rebuild_TR verifies ΔBTree state after Track(keys) + rebuild.
+func xverifyΔBTail_rebuild_TR(t *testing.T, db *zodb.DB, δbtail *ΔBtail, tj *tTreeCommit, treeRoot zodb.Oid, xat map[zodb.Tid]string, keys SetKey, trackIdx trackIndex, trackNew, trackIdxAfterRebuild trackIndex,vδBok []ΔB) {
+	t.Helper()
+	X := exc.Raiseif
+	ø := trackIndex{}
 
 	// Track(keys)
 	txn, ctx := transaction.New(context.Background())