.

wcfs/internal/xbtree/treediff.go

@@ -396,7 +396,8 @@ func treediff(ctx context.Context, root zodb.Oid, δtops setOid, δZTC setOid, t
 // δtkeycov represents how δtrack grows (always grows) tracking set key coverage.
 func diffX(ctx context.Context, a, b Node, δZTC setOid, trackSet blib.PPTreeSubSet) (δ map[Key]ΔValue, δtrack *blib.ΔPPTreeSubSet, δtkeycov *blib.RangedKeySet, err error) {
 	if a==nil && b==nil {
-		panic("BUG: both a & b == nil")	// XXX -> not a bug e.g. for `ø ø T` sequence?
+		// DEL..DEL -> ø diff
+		return map[Key]ΔValue{}, blib.NewΔPPTreeSubSet(), &blib.RangedKeySet{}, nil
 	}
 
 	var aT, bT *Tree