src/Pure/General/file.scala

     write_xz(path.file, text, options)
   def write_xz(path: Path, text: CharSequence): Unit = write_xz(path, text, XZ.options())
 
   def write_backup(path: Path, text: CharSequence)
   {
-    mv(path, path.backup)
+    if (path.is_file) mv(path, path.backup)
     write(path, text)
   }
 
   def write_backup2(path: Path, text: CharSequence)
   {
-    mv(path, path.backup2)
+    if (path.is_file) mv(path, path.backup2)
     write(path, text)
   }
 
 
   /* append */