257 lines
9.5 KiB
Plaintext
257 lines
9.5 KiB
Plaintext
This document describes the OBJECT-INTERPRETATION attribute.
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This attribute resides in the APPLICATION namespace. It is of type
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SEQUENCE. It normally has OBJECT precedence.
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There may be multiple instances of the OBJECT-INTERPRETATION attribute.
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The first token specifies the class of the object. The class tells
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the user of a simple-minded browser what you can DO with the object.
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It tells the browser what icon to put up.
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In general, the second token is interpreted on a per-class basis as
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the particular format in use. The third and subsequent tokens depend
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upon the file format in use.
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(For the Prospero menu browsers, the icons we put up currently are :
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for searches, . for files, ] for portals, and > for directories).
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The classes are:
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.. UNKNOWN This will never be sent over the network. It is used
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internally by clients.
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.. VOID
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This class is used when the TARGET of a link is NULL. The only cases
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in which this arises are (A) when a link is listable but not readable
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(then the server's LIST command returns this link) and (b) when a
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gopher type of 'i' is sent. (This is a little-used type supported by
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Panda, the University of Iowa Gopher variant.)
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A menu browser should just display this with no type character -- it's
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a pure text string. All attempts to retrieve it will fail. Just look
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at the MENU-ITEM-DESCRIPTION and NAME.
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A server should never return such a link with any attributes except
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(possibly) an OBJECT-INTERPRETATION of VOID.
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.. PORTAL -- Portals are always attached to raw OBJECTs (not yet
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implemented) or to EXTERNAL links. They will NEVER be attached to
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FILEs or DIRECTORYs, although the implementor should not bother
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checking this fact. One looks at the ACCESS-METHOD to determine how
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to use it.
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An OBJECT-INTERPRETATION of portal means that the process of accessing
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the object leaves the user in some sort of interactive session.
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. Details
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PORTALS: For now, the only implementation of the PORTAL class is to
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have the TELNET access method on an object. This instance of the
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ACCESS METHOD token has type TELNET, and that is the 1st token. Of
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the conventional next 4 arguments, only HOST is relevant. The hsoname
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is ignored. The fifth argument is a set of instructions to display to
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the user before opening the connection. If the fifth argument is not
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present, that is the same as if it is explicitly specified as the null
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string. We will later extend that to a full template, so anything
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that parses this access method should ignore any additional arguments
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beyond the fifth for now; they may be used later.
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The conventional 4 arguments to an ACCESS METHOD are that the 2nd
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token is the host type (always INTERNET-D for now), the 3rd is a
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hostname, optionally followed by a port # in parentheses (in the case
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of the TELNET access method, the port # is mandatory), the 4th is the
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HSONAME type (always ASCII) and the fifth is the HSONAME (ignored in
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this case)
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If the port # is not specified in the HOST name, then the default port
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for the TELNET service is used.
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.. Gopher mapping
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This can be served up by a Gopher server as a gopher menu/directory
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containing two entries: (a) the directions, as a Gopher file. (b) the
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TELNET link.
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A Prospero server that makes Gopher protocol queries should serve up
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the Gopher type '8' (Telnet session) as an objet with PORTAL class and
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TELNET access method. If a GOPHER selector string (username) is
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present, the instructions should be the literal string:
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'Use the account name "<selector-string>" to log in'
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This convention will guarantee that information transformed from
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Gopher Protocol to Prospero format will be able to automatically be
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transformed back into Gopher protocol.
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The TN3270 access method is identical to the TELNET access method,
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except that the word TELNET is replaced by TN3270 in the above
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description. Gopher type 'T' should be served up as the TN3270 access
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method. If the HOST does not specify a port, then the default port
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should be used.
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.. EXECUTABLE -- At the moment, the only format (2nd token) firmly
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defined is PRM. The third token is always PRM_V1. We propose
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EXECUTABLE SUN4 and EXECUTABLE SUN3, but this is subject to change
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depending on implementation experience. The third and subsequent
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tokens are not clear (SUNOS4, perhaps?).
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Treat it as DATA if you don't recognize the format.
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.. DOCUMENT
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At this moment, the only format is TEXT. The third token is ASCII;
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other character sets will be defined later.
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DOCUMENT MIME is defined but won't be implemented immediately. The
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GOPHER 'M' type will be exported under Prospero as DOCUMENT MIME.
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If you don't have a MIME reader, treat it as DOCUMENT TEXT ASCII.
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.. SEARCH --
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Only form for this: SEARCH QUERY-METHOD V1.
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A long form of the query-method attribute is currently defined in
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/pfs/notes/query-method. This will be SEARCH QUERY-METHOD V2, and
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will be a superset of V1. For V1, we are just implementing the simple
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Gopher search type. Look at how to do this in the file
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'/pfs/notes/gopher-searches', under the section 'How to handle this
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under Prospero'.
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If no QUERY-METHOD attribute is found, this can be handled at your
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discretion. One reasonable way to handle it would be to quietly (not
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obnoxiously) warn the user that the link has an erroneous format.
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Don't go to the expense of more protocol requests to retrieve the
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QUERY-METHOD until the link is actually selected.
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This corresponds to gopher type '7' and will correspond to type '2'
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when I implement the CSO gateway.
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.. VIRTUAL-SYSTEM
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The only format for this CLASS is VS-DESCRIPTION. The VIRTUAL-SYSTEM
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VS-DESCRIPTION format is only associated with directories.
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Menu browser should treat it as a DIRECTORY for now.
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.. SOUND
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No format defined at the moment.
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Corresponds to gopher type 'S' and Gopher+ type '<'.
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Treat it as DATA if you don't have a sound player or don't recognize
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the format.
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.. IMAGE
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Only format currently in use: IMAGE GIF. IMAGE POSTSCRIPT is defined;
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an object may be DOCUMENT POSTSCRIPT or IMAGE POSTSCRIPT, depending on
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whether it is primarily considered an image or a document.
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MPEG, JPEG, X bitmaps, and other image encodings will be added later
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as needed.
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IMAGE GIF corresponds to the gopher 'g' type.
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Treat it as DATA if you don't have an image viewer.
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.. VIDEO
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The VIDEO class may contain prerecorded (perhaps live too? We'll have
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to see as a result of use) sound and image data. Treat it as DATA for now.
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.. SOURCE-CODE
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The MENU API may treat this as identical to DOCUMENT TEXT ASCII. The
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file-format (2nd field) is the computer language it's source code for.
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The current defined computer language is 'C'.
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.. DATA
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This is for raw files that just contain raw binary data. No
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file-format is defined for them. They cannot be displayed or printed.
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Just offer to save them.
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.. PROGRAM
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Interpreted code will be of class "PROGRAM", which means you can
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execute it using the interpreter named in the format field, and you
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can display it as an ASCII document. No official interpreter names
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are currently defined for PROGRAM, although we will come up with some
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on an as-needed basis. I suggest the first interpreter names be 'PERL',
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'SED', 'SH', and 'CSH'. This class is for things which are really
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both SOURCE-CODE and EXECUTABLE.
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.. EMBEDDED
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This class is used for objects that have to be uncompressed, unzipped,
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untarred, or have some other transformation applied to them to be
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useful.
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The 2nd token (format) should identify the transformation. Currently
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defined ones: GZIP, COMPRESS, TAR, BINHEX (macintosh BinHex format),
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UUENCODE.
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The third field for any transformation of class EMBEDDED may be a
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number. If it is, it indicates the number of additional arguments to
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be specified for the transformation. If a number is specified, that
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many arguments follow. Any tokens after the arguments are parsed as
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if they were a new freestanding OBJECT-INTERPRETATION attribute. If
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this third field is not a number, then it is assumed to be zero, and
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the third and subsequent tokens should be parsed as if they were a
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freestanding OBJECT-INTERPRETATION attribute.
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GZIP, COMPRESS, TAR, BINHEX, and UUENCODE are not currently defined to
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take additional arguments. If a specific file is to be extracted from
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the tar file, see the SEGMENT field (as defined in
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/pfs/notes/aggregate).
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Default for saving should be to save it in the raw (unextracted) form.
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A more sophisticated client can offer to unembed it before saving it.
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If you don't recognize a particular EMBEDDED type, just treat it as
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DATA. You can still offer to save it in a file.
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.. AGGREGATE
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This type is documented in another file but does not need to be
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discused here (yet).
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.. DIRECTORY
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This is a raw DIRECTORY with no special OBJECT-INTERPRETATION.
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. Kwynn:
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For now, just implement the following types:
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PORTAL (with the ACCESS-METHOD TELNET)
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DOCUMENT TEXT ASCII
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SEARCH
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DIRECTORY
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DATA
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Your next pass will be to implement:
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VOID
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IMAGE GIF
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EMBEDDED GZIP
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EMBEDDED COMPRESS
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Until you implement all of the OBJECT-INTERPRETATIONs, please do this hack:
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EXECUTABLE, AGGREGATE, IMAGE, SOUND, EMBEDDED, and DATA should all be
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treated as raw DATA.
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Treat DOCUMENT MIME, SOURCE-CODE, and PROGRAM as if they were DOCUMENT
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TEXT ASCII.
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Treat VIRTUAL-SYSTEM as if it were just DIRECTORY.
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. If No OBJECT-INTERPRETATION present
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If no OBJECT-INTERPRETATION is present, treat random FILEs as if
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they were either DOCUMENT TEXT ASCII or DATA. Just display them as
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TEXT, but when you're asked to retrieve them, sample the first 1k or
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so and make sure it's printable. If it isn't, say that the file isn't
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really text, and offer to save it.
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Treat unadorned directories as if they were DIRECTORY.
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