.

wcfs/internal/xbtree/δbtail.go

@@ -516,15 +516,13 @@ func (δBtail *ΔBtail) Update(δZ *zodb.EventCommit) (_ ΔB, err error) {
 			δTtail.trackNew = nil // XXX stub
 
 			// XXX vvv we can skip computing diff for HEAD~..HEAD (we just
-			//     computed it)
+			//     computed it in _Update1)
 			δtrackSet, err := δTtail.rebuild(root, δBtail.δZtail, δBtail.db)
 			if err != nil {
 				return ΔB{}, err
 			}
 
 			δBtail.trackSet.UnionInplace(δtrackSet)
-			// XXX Update δTKeyCov? XXX or it is needed only to be returned
-			//     from treediff?
 		}
 
 		// build δB. Even if δT=ø after _Update1, but δtkeycov1 != ø, above

wcfs/internal/xbtree/δbtail_test.go

@@ -806,6 +806,9 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 
 
 	// δB <- δZ
+	//
+	// 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()
 	δB, err       := δbtail.Update(δZ);     X(err)
 	δTKeyCov, err := δbtail_._Update1(δZ);  X(err)