X another bug in δtkeyconv computation

=== RUN   TestΔBTail/Update/T7/B1:a,2:b-B8:c,9:d→T3/T-T/B1:a,2:b-B8:c,9:d
    δbtail_test.go:832: 2: trackSet:
                have: map[000000000000000c:{pffffffffffffffff c2} 0000000000000041:{p0000000000000043 c0} 0000000000000042:{p0000000000000044 c0} 0000000000000043:{p000000000000000c c1} 0000000000000044:{p000000000000000c c1}]
                want: map[000000000000000c:{pffffffffffffffff c1} 0000000000000041:{p0000000000000043 c0} 0000000000000043:{p000000000000000c c1}]
    δbtail_test.go:708: T7/B1:a,2:b-B8:c,9:d -> T3/T-T/B1:a,2:b-B8:c,9:d  ; tracked={1} :

        d12:  map[]
        δTok: map[]
        δT:   map[]

        δZ:               {000000000000000c 0000000000000043 0000000000000044}
        Tracked^δZ:       {}
        kadj[Tracked^δZ]: {}
        kadj: map[1:{1 2 8 9 9223372036854775807} 2:{1 2 8 9 9223372036854775807} 8:{1 2 8 9 9223372036854775807} 9:{1 2 8 9 9223372036854775807} 9223372036854775807:{1 2 8 9 9223372036854775807}]

        δtkeycov wrong:
        have: {[7,∞)}
        want: {}

wcfs/internal/xbtree/δbtail.go

@@ -376,6 +376,7 @@ func (δTtail *ΔTtail) rebuild(root zodb.Oid, δZtail *zodb.ΔTail, db *zodb.DB
 }
 
 // widenTrackNew widens trackNew to cover δtkeycov.
+// XXX -> widenTrackSet?
 func widenTrackNew(trackNew PPTreeSubSet, δtkeycov *RangedKeySet, root zodb.Oid, at zodb.Tid, db *zodb.DB) error {
 	// XXX errctx, debug
 

wcfs/internal/xbtree/δbtail_test.go

@@ -804,7 +804,7 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 	// also call _Update1 directly to verify δtkeycov return from treediff
 	// the result of Update and _Update1 should be the same since δbtail is initially empty.
 	δbtail_ := δbtail.Clone()
-	δB1, err := δbtail_._Update1(δZ);  X(err)
+	δB1, err := δbtail_._Update1(δZ);  X(err)	// XXX don't compute treediff twice
 
 	// assert δtkeycov == δ(tkeyCov1, tkeyCov2)
 	δtkeycovOK := tkeyCov2.Difference(tkeyCov1)
@@ -1684,6 +1684,11 @@ func TestΔBTail(t *testing.T) {
 		"T3/B1:a,2:b-B8:c,9:d",
 		"T7/B1:a,2:b-B8:c,9:d",
 
+		// ----//---- depth=2
+		"T3/T-T/B1:a,2:b-B8:c,9:d",
+		"T7/T-T/B1:a,2:b-B8:c,9:d",
+
+
 
 //		"T/T/T/B1:a,2:b",
 //		"T/T/B1:a",