--- a/src/xml.rs	Sun Nov 22 08:48:12 2020 +0900
+++ b/src/xml.rs	Sun Nov 22 09:04:24 2020 +0900
@@ -160,6 +160,7 @@
                     writer.write_event(Event::Empty(elem)).ok();
                 }
 
+                // use template qcow2
                 Ok(Event::End(ref e)) if ((e.name() == b"disk") && self.backing_file.is_some()) => {
                     let mut backing_store_start = BytesStart::borrowed_name(b"backingStore");
                     backing_store_start.push_attribute(("type", "file"));
@@ -173,7 +174,7 @@
                     writer.write_event(Event::Empty(format_elem)).unwrap();
 
                     let mut backing_sorce = BytesStart::borrowed_name(b"sorce");
-                    let backing_file: &str = &self.backing_file.clone().unwrap();
+                    let backing_file: &str = self.backing_file.as_ref().unwrap();
                     backing_sorce.push_attribute(("file", backing_file));
                     writer.write_event(Event::Empty(backing_sorce)).unwrap();
 
@@ -181,6 +182,7 @@
                     writer.write_event(Event::End(backing_store_end)).unwrap();
                 }
 
+                // replace qcow2 file
                 Ok(Event::Empty(ref e)) if (e.name() == b"source") => {
                     let mut elem = e.clone();
                     let is_qcow_file = elem