LaTeX.org

Problem solved. sequential index number is AUTO supplied.

I reset my backup file name format as
*\_history\%n%t.bak //my format spec
But then I couldn't see the sequential number <sn> as in
<file name>.<file type>.<sn>.bak

where <sn> may take values as ~0,~1, etc.

What should I add or change to my format spec above?
Actually I feel awkward to express ...