X vδT should not keep empty entries

wcfs/δbtail_test.go

@@ -1015,14 +1015,20 @@ func xverifyΔBTail_rebuild_TR(t *testing.T, db *zodb.DB, δbtail *ΔBtail, tj *
 	// verify SliceByRootRev	XXX -> verify δbtail.byRoot[treeRoot] directly
 	// XXX +KVAtTail, +lastRevOf
 	l := len(vδTok)
-	vatOK := make([]zodb.Tid, l)
+	vatOK  := []zodb.Tid{}
+	vδTok_ := []map[Key]Δstring{}
 	t0 := tj
 	for i := 0; i<l; i++ {
-		vatOK[l-i-1] = t0.at
+		// empty vδTok entries means they should be absent in vδT
+		if δTok := vδTok[l-i-1]; len(δTok) != 0 {
+			vatOK  = append([]zodb.Tid{t0.at}, vatOK...)
+			vδTok_ = append([]map[Key]Δstring{δTok}, vδTok_...)
+		}
 		t0 = t0.prev
 	}
+	vδTok = vδTok_
 	lo := t0.at
-	hi := vatOK[l-1]
+	hi := tj.at
 	vδToid := δbtail.SliceByRootRev(treeRoot, lo, hi)
 
 	l = len(vδToid)