X random testing found another failing test for rebuild...

https://nexedijs.erp5.net/#/test_result_module/20210816-95C3A93E/5

wcfs/internal/xbtree/δbtail_test.go

@@ -387,6 +387,11 @@ func TestΔBTail(t *testing.T) {
 			  3:  K(3,99,oo),
 			  99: K(3,99,oo),
 			  oo: K(3,99,oo)}),
+
+		// XXX wrong rebuild
+		"T4/T1,3-T/T-T-T-T/B0:b-B1:c,2:j-T-B4:d/B3:h",
+		"T/T2,3/T-T-T/B1:d-B2:c-B3:i",
+		"T2/B1:g-B2:c,3:i",
 	}
 	// direct  tree_i -> tree_{i+1} -> _{i+2} ...   plus
 	// reverse ... tree_i <- _{i+1} <- _{i+2}