The structure FILE-MODE implements a data type for file modes. The
code is copied verbatim from Scheme 48 0.57.

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After installation, use the switch

-lel file-mode/load.scm

to load this library.

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A file mode is a boxed integer representing a file protection mask.

    * (file-mode permission-name ...) -> file-mode 	syntax
    * (file-mode? x) -> boolean 
    * (file-mode+ file-mode ...) -> file-mode 
    * (file-mode- file-mode0 file-mode1) -> file-mode  

FILE-MODE is syntax for creating file modes. The mode-names are listed
below. FILE-MODE? is a predicate for file modes. FILE-MODE+ returns a
mode that contains all of permissions of its arguments. FILE-MODE-
returns a mode that has all of the permissions of FILE-MODE0 that are
not in FILE-MODE1.

    * (file-mode=? file-mode0 file-mode1) -> boolean 
    * (file-mode<=? file-mode0 file-mode1) -> boolean 
    * (file-mode>=? file-mode0 file-mode1) -> boolean  

FILE-MODE=? returns true if the two modes are exactly the
same. FILE-MODE<=? returns true if FILE-MODE0 has a subset of the
permissions of FILE-MODE1. FILE-MODE>=? is FILE-MODE<=? with the
arguments reversed.

    * (file-mode->integer file-mode) -> integer 
    * (integer->file-mode integer) -> file-mode  

INTEGER->FILE-MODE and FILE-MODE->INTEGER translate file modes to and
from the classic Unix file mode masks. These may not be the masks used
by the underlying OS.

    Permission name 	Bit mask
    set-uid 	#o4000 	set user id when executing
    set-gid 	#o2000 	set group id when executing
    owner-read 	#o0400 	read by owner
    owner-write 	#o0200 	write by owner
    owner-exec 	#o0100 	execute (or search) by owner
    group-read 	#o0040 	read by group
    group-write 	#o0020 	write by group
    group-exec 	#o0010 	execute (or search) by group
    other-read 	#o0004 	read by others
    other-write 	#o0002 	write by others
    other-exec 	#o0001 	execute (or search) by others

    Names for sets of permissions
    owner 	#o0700 	read, write, and execute by owner
    group 	#o0070 	read, write, and execute by group
    other 	#o0007 	read, write, and execute by others
    read 	#o0444 	read by anyone
    write 	#o0222 	write by anyone
    exec 	#o0111 	execute by anyone
    all 	#o0777 	anything by anyone