The following text is excerpted verbatim from "An Introduction to ITS for the
MACSYMA User" by Ellen Golden, as revised April 14, 1981.

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IV. Introduction to and Description of Files

A file is simply an area on a particular storage device which contains data or
text.  The only storage devices which are used on the MC machine are disks and
tapes.

Files on the disks are figuratively grouped into "directories".  A directory is
just a list of all the files stored under a given name.


IV.A. File Names

Files in the ITS system possess two names, each of at most 6 characters.  They
are referenced by giving the two names as well as the device (default is DSK:)
and the directory name where the file resides.  A device is usually a machine
name, meaning the disk of that machine.  The device name DSK: refers the disk
of the machine you are using.  Thus on MC, either DSK: or MC: means MC's disks.
A device name is always followed by a colon.

A directory name is always followed by a semi-colon.  Your directory is
determined when you login: if you have a directory of your own, its name will
be the same as your login name; otherwise it is the default specified in your
INQUIR entry, if one was specified; if none was specified, you will be assigned
by the system to a "home directory" called USERSn; where n is a digit from 0 to
9.  You may determine which directory you have been assigned to by typing

        :HSNAME


File names are up to six characters long and separated by spaces.  A > sign may
be used for the second file name and stands for the name which is the largest
numerically if there exists a file with the given first name and a numeric
second name; otherwise the > sign represents the "greatest" name in an
alphanumeric sense.  A < sign may be similarly used for the "least" name.

The device (followed by a colon), the directory name (followed by a semicolon),
and the file names (separated by a space) may appear anywhere in the file
descriptor string.  Thus for example both

        INTRO 4 DSK:RAB;
        RAB;DSK:INTRO 4

are equivalent descriptions.  However, the most usual description is

        DSK:RAB;INTRO 4

Any of the four descriptors (1st name, 2nd name, device, directory) may be
omitted and either the default, or the value given in a previous command (if
there was one given) will be used.  Notice that at DDT level the "usual" way to
reference this file is:

        DSK:RAB;INTRO 4

while in MACSYMA the two file names are given first and then the device and
directory,

        (INTRO, 4, DSK, RAB).

i.e. (filename1,filename2,device,directory), which corresponds to the DDT
ordering:  INTRO 4 DSK:RAB;.