archive-calc-mode is a compiled Lisp function in `
(archive-calc-mode OLDMODE NEWMODE &optional ERROR)
From the integer OLDMODE and the string NEWMODE calculate a new file mode.
NEWMODE may be an octal number including a leading zero in which case it
will become the new mode.
NEWMODE may also be a relative specification like "og-rwx" in which case
OLDMODE will be modified accordingly just like chmod(2) would have done.
If optional third argument ERROR is non-nil an error will be signaled if
the mode is invalid. If ERROR is nil then nil will be returned.