With the keyword -backup the command gives
a backup file if the same name file has already existed as that
for saving. The backup file name is attached by "." and number digits
which correspond the version number for backup attached with "+".
For example, "abcd.scn" is renamed "abcd.0+.scn".
The backup version number automatically increases
when the save command is executed.
With the keyword -backup the command does not
make a backup file.