Network Working Group                                        L. Masinter
Internet-Draft                                             Adobe Systems
Intended status: Informational                          February 3, 2009
Expires: August 7, 2009

          "duri" and "tdb" URN namespaces based on dated URIs

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   This document defines two namespaces of URNs, based on using a
   timestamp with an (encoded) URI.  The results are namespaces in which

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   names are readily assigned, offer the persistence of reference that
   is required by URNs, but do not require a stable authority to assign
   the name.  The first namespace ("duri") is used to refer to URI-
   identified resources as they appeared at a particular time.  The
   second namespace ("tdb") is useful as a way of creating URNs that
   refer to physical objects or even abstractions that are not
   themselves networked resources.

   The definition of these namespaces may reduce the need to define new
   URN namespaces merely for the purpose of creating stable identifiers.
   In addition, they provide a ready means for identifying "non-
   information resources" by semantic indirection.


   This document is not a product of any working group.  Many of the
   ideas here have been discussed since 2001.  This document has been
   discussed on the mailing list <>.

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Table of Contents

   1.  Overview and Requirements  . . . . . . . . . . . . . . . . . .  4
     1.1.  Easy URN assignment  . . . . . . . . . . . . . . . . . . .  4
     1.2.  Persistent identifiers . . . . . . . . . . . . . . . . . .  4
     1.3.  URIs for abstractions  . . . . . . . . . . . . . . . . . .  5
   2.  Namespace definitions  . . . . . . . . . . . . . . . . . . . .  6
     2.1.  "duri" namespace . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  "tdb" namespace  . . . . . . . . . . . . . . . . . . . . .  6
   3.  Encoding URIs  . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Characters that must be encoded  . . . . . . . . . . . . .  7
     3.2.  Hierarchy and unencoded /  . . . . . . . . . . . . . . . .  8
   4.  Dates  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Additional Considerations  . . . . . . . . . . . . . . . . . .  8
     5.1.  Embedded URI schemes . . . . . . . . . . . . . . . . . . .  9
     5.2.  Why dates for both tdb and duri? . . . . . . . . . . . . . 10
     5.3.  Useful dates . . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Free assignment  . . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Resolution . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.6.  Why Names with Semantics?  . . . . . . . . . . . . . . . . 11
     5.7.  Avoiding MetaData  . . . . . . . . . . . . . . . . . . . . 11
     5.8.  Avoiding duri and tdb  . . . . . . . . . . . . . . . . . . 11
     5.9.  tdb and levels of indirection  . . . . . . . . . . . . . . 12
   6.  URN Specification Templates  . . . . . . . . . . . . . . . . . 12
     6.1.  duri Specification Template  . . . . . . . . . . . . . . . 12
     6.2.  tdb Specification Template . . . . . . . . . . . . . . . . 13
   7.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15

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1.  Overview and Requirements

   The URN namespaces defined here solve several related problems.

1.1.  Easy URN assignment

   The URN specification [7] allows for many URN namespaces, and many
   have been registered.  However, obtaining an appropriate URN in any
   of the currently defined URN namespaces may be difficult: a number of
   URN namespace registrations have been accompanied by comments that no
   other URN namespace was available for the class of documents for
   which identifiers were wanted.

1.2.  Persistent identifiers

   RFC 1737 [7] defines several requirements for Uniform Resource Names.
   In particular, it requires "persistence":
      Persistence: It is intended that the lifetime of a URN be
      permanent.  That is, the URN will be globally unique forever, and
      may well be used as a reference to a resource well beyond the
      lifetime of the resource it identifies or of any naming authority
      involved in the assignment of its name.

   Many people have wondered how to create globally unique and
   persistent identifiers.  There are a number of URI schemes and URN
   namespaces already registered.  However, an absolute guarantee of
   both uniqueness and persistence is very difficult.

   In some cases, the guarantee of persistence comes through a promise
   of good management practice, such as is encouraged in "Cool URLs
   don't change" [6].  However, relying on promise of good management
   practice is not the same as having a design that guarantees
   reliability independent of actual administrative practice.

   A primary design goal for URIs is that they are intended to mean the
   same thing, no matter in what context they appear: a "Uniform" way to
   Identify a Resource.  However, even when URIs have Uniform meaning
   from the point of view of the source of the reference, they don't
   guarantee stability over time.  Despite best efforts and intentions,
   identifying information can change in unpredictable ways: domain
   names can disappear or be reassigned, name assigning organizations
   can change structure, responsibility, disappear, merge, or change in
   unpredictable ways.

   There is a significant dependence in the interpretation of many URNs
   with the concept of "naming authority".  The authority is presumably
   some individual or organization both to insure uniqueness of
   assignment and also to help with understanding the meaning of the

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   link between the name and the named.

   However, authorities, whether individuals or organizations, have a
   lifetime, and must be consulted at some point to understand the
   bindings.  The functioning of names as unique identifiers and holders
   of meaning depends on having a reliable infrastructure of consulting
   the authority or the authorities records to determine the thing

1.3.  URIs for abstractions

   The description of URIs [3] describes a range for 'Resource' that is
   quite broad:
      This specification does not limit the scope of what might be a
      resource; rather, the term "resource" is used in a general sense
      for whatever might be identified by a URI.  Familiar examples
      include an electronic document, an image, a source of information
      with a consistent purpose (e.g., "today's weather report for Los
      Angeles"), a service (e.g., an HTTP-to-SMS gateway), and a
      collection of other resources.  A resource is not necessarily
      accessible via the Internet; e.g., human beings, corporations, and
      bound books in a library can also be resources.  Likewise,
      abstract concepts can be resources, such as the operators and
      operands of a mathematical equation, the types of a relationship
      (e.g., "parent" or "employee"), or numeric values (e.g., zero,
      one, and infinity).

   One might use a URI such as "mailto:" email address to identify a
   person, or a "http:" URI to identify an abstract comment.  However,
   this leaves the question of how one might identify, within the same
   context, both the system mailbox and the person to which it is
   assigned, or the web page at a http URI and the person or concept it
   describes.  The "tdb" URN scheme allows ready assignment of URIs for
   abstractions that are distinguished from the media content that
   describes them.

   The goal, then, of the "tdb" URN scheme proposed below is to provide
   a mechanism which is, at the same time:
      permanent: The identity of the resource identified is not subject
      to reinterpretation over time.
      explicitly bound: The mechanism by which the identified resource
      can be determined is explicitly included in the URI.
      useful for non-networked items: Allows identification of resources
      outside the network: people, organizations, abstract concepts.

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      no administration: The mechanism does not depend on reliable
      administrative processes of authorities for either assignment or

2.  Namespace definitions

2.1.  "duri" namespace

   It is traditional in convention references and citations in printed
   works to include the date of publication; this practice serves the
   important purpose that the context of the naming can be determined.

   The "duri" URN namespace takes the form:

   where <date> is a digit string corresponding to a date (Section 4),
   and an <encoded-URI> is an absolute URI-reference [3] in which any
   character excluded from URN syntax has been escaped (Section 3).

   The meaning of a duri is "the resource (or fragment) that was
   identified by the <encoded-URI> (after hex decoding) at the very last
   instant of the date(time) given".

   For example, 'urn:duri:2001:' is a persistent
   identifier to '' as of the very last instant of
   the year 2001.  A duri may not be a resource locator in a practical
   sense if the time of location has not yet arrived or has passed.
   However, is an acceptable resource identifier, and fulfills all of
   the requirements for URNs [7].

2.2.  "tdb" namespace

   The second URN namespace defined is a parallel space which is useful
   for describing entities, concepts, abstractions, and other items
   which are not themselves network accessible resources, but have been
   at some point described by network accessible resources.

   The "tdb" namespace designates the "thing described by" a resource at
   a given URI at the given time.  This URN namespace is described by
   'tdb', e.g.,


   with the same syntactic rules as 'duri'.

   The intent is to use the inversion of "is a document about".  It is
   common practice to give a reference for a concept by including a

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   pointer to a document, segment, phrase that defines the concept.
   "tdb" attempts to capture this practice in URI space.

   For example, "urn:tdb:2001:" can be used to
   designate the Internet Engineering Task Force organization, at least
   as it was described by or referenced by its home page at the last
   instant of 2001.

   The "tdb" namespace differs from most other URN methods for
   identifying abstractions because the designation of what is actually
   identified by the tdb doesn't depend on knowing the intention of the
   "assigner" of the identifier.  Unlike many of the alternatives
   proposed, the identification is not dependent on the context of use.

   The "tdb" namespace can be thought of as adding a level of semantic
   indirection to URI resolution.  While one could imagine using 'tdb'
   without a date, it would leave the possibility that a reference that
   is unambiguous at one time might become ambiguous at some other time.
   There are two ways that the date is useful for "tdb" URNs: it fixes
   the time of access of the resource, for variable descriptions, and it
   fixes the time of interpretation, for descriptions whose meaning (in
   natural language) might vary.

3.  Encoding URIs

   Both "duri" and "tdb" URN namespaces require that some characters in
   the URI references be encoded.

3.1.  Characters that must be encoded

   The characters that must be encoded are:
   o  All characters marked excluded in RFC 2141 [1], section 2.4:
      These are excluded because they are not allowed in URNs.
   o  The character "#"
      Note that <encoded-URI> can include a fragment identifier; the "#"
      character used to delimit it must be encoded.  This feature is
      intended for use with "tdb", where the fragment identified might
      contain the description.  Including an encoded "#" with a "duri"
      is not as useful, since the fragment identifier might as well be
      applied to the duri itself.
   o  The character "%"
      The encoded-URI can itself contain encoded characters, which are
      encoded with the same method.  To insure that decoding happens at
      the right level of processing, the "%" itself must be encoded.
      Unfortunately, there may well be cases where there is a double
      encoding of characters, first to construct the embedded URI itself

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      and second to then embed the URI within the tdb or duri URN.

3.2.  Hierarchy and unencoded /

   The URN specification [7] discourages the use of "/" in URNs because,
   in general, there is no good interpretation of hierarchy and relative
   URIs for assigned names.  However, for the particular case of duris
   (at least), there seems to be no good reason to avoid the "/" because
   it corresponds fairly naturally (in many cases) to the hierarchy of
   the original space.

   Note that because of this, "duri" URNs can actually be used with
   relative URI references with some amount of reliability.  (The double
   encoding of previously encoded URI characters causes some problems.)

4.  Dates

   A <date> is a simple expression of date, optional time, with
   arbitrary precision.  The goal is to allow relatively short
   expressions of dates with no ambiguity, and with arbitrary precision.

     date = year [ month [ day [ hour [ minute [ second [ fraction ]]]]]]

      year     = 4digit
      month    = 2digit
      day      = 2digit
      hour     = 2digit
      minute   = 2digit
      second   = 2digit
      fraction = *digit

   The representation of a date or time refers to the very last instant
   of the given date/time range at the resolution supplied, so that 1999
   and 1999123123595999999 are equivalent.  If necessary, "dates" can
   include times and even fractional times, so that a generator of duris
   can be arbitrarily precise.

   Dates are interpreted relative to International Atomic Time (TAI)
   [4].  The syntax and semantics are similar to those in RFC 2550 [8];
   in particular, using TAI avoids ambiguity about time zones and
   difficulties with leap seconds.

5.  Additional Considerations

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5.1.  Embedded URI schemes

   The intent of "duri" and "tdb" is to use them with embedded URI
   references that identify documents or document fragments.  That is,
   they are most useful for information resources.

   For example, use with a "http" URI can be used to refer to a web page
   or the subject of a web site at a given time.  This can be a way of
   referring to a web site at some time in the past, or an organization
   that has changed or merged.

   Local systems that have known-to-be unique host names can use "file"
   URIs with "tdb", for example,


   since this use is primarily focused on providing a unique way of
   identifying an abstraction, even if the referent of the abstraction
   is not widely known.  (Using 'file:' URIs in this way without a fully
   qualified domain name is not recommended, because the interpretation
   is not uniform.)

   Some URI schemes are more problematic.  For example, using tdb or
   duri with an embedded 'urn:' might not seem to be too useful.  But it
   might be useful where the assignment of names in a URN namespace are
   not, in practice, permanent, or that one might want to refer to the
   assignment as of a given date.  In this case, it is possible to use a
   "urn" within a "duri", e.g.,


   might be used to refer to "the document that was STD 50 in effect as
   of the last instant of 2000". [2]

   One might consider using "tdb" with "data" to designate concepts that
   can be described uniquely briefly inline.  For example,


   names the concept described by the (text/plain) string "The US
   president" at the very last instant of 2001.  (Note the awkward
   double quoting of space as "%20" and then the "%" as "%25".)  Of
   course, this practice is only useful if the referent of the data is
   (or was at the time) completely unique.  Since "data" does not
   contain a way to designate content-language, the string in question
   would have to not be ambiguous as to its language.  In the case of
   'data', there is no assigning authority at all; the interpretation of
   the 'tdb' URN depend on the interpreting community.

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5.2.  Why dates for both tdb and duri?

   Some have suggested that perhaps only one of "tdb" or "duri" might
   have a date parameter, e.g., allow "tdb:<encoded-URI>" as a URI
   scheme, and "urn:duri:<date>:tdb:<encoded-URI>" if one really wanted
   to fix the date of interpretation.  While doing this might be
   possible, it seems that the usage pattern which allows the date to be
   omitted when needed satisfies the instances where the resource doing
   the description is permanent to stand.

   There are actually two dates to consider, with "tdb".  There is the
   date that the resource is obtained, and there is the date that the
   description it makes is read, understood, and used to denote.
   Normally in a literary work in natural language which makes a
   reference to another work, both the reference itself and the work
   referenced are dated, e.g., a footnote in an article written in 1967
   might talk about a "private communication" which itself had a date.
   The difference between a URI and a conventional literary reference is
   the desire to be able to extract the URI from its context and still
   retain its meaning.

5.3.  Useful dates

   Dates far in the future are suspect, because the meaning of the duri
   or tdb cannot readily be determined in advance reliably.  Dates whose
   range ends before assignment of the resource to the embedded URI
   SHOULD NOT be used, because the meaning of the reference is left in
   question.  For example, using http URIs with a date range which ends
   before a web service was available at the given URI doesn't make much

   However, although these practices are NOT RECOMMENDED, there is no
   assurance that they haven't been used; by itself, a duri/tdb does not
   constitute an assertion that the encoded-URI was available or
   assigned at the date specified.

   Note that the use of the "very last instant" allows for the
   conventional bibliographic convention that a work published in 2009
   can use "2009" as the date string, to refer to the work in the year
   of publication.

5.4.  Free assignment

   Because of the many possible schemes that can be used in the
   <encoded- URI> portion, there should be no difficulty in almost any
   computational process being able to assign duris or tdbs at will.  Of
   course, it is necessary for there to be some resource which is
   available at some point in time, and to have a clock which is

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   accurate to the granularity of the frequency of assignment.

5.5.  Resolution

   There no accurate resolution servers for duri or tdb URNs.  However,
   duri might be "resolvable" in the sense that a resource that was
   accessed at a point in time might have the result of that access
   cached or archived in an Internet archive service.  See, for example,
   the "Internet Archive" project [9].  A "tdb" is only tesolvable in
   the sense that if the corresponding duri can be esolved, it may be
   possible that the result can be accessed and interpreted.

   Clients without access to an Internet archive service might take the
   decoded <encoded-URI> of a duri and attempt resolution of *that*
   identifier.  This will give an approximation whose reliability
   depends on the amount of time elapsed since the date indicated.

5.6.  Why Names with Semantics?

   There are a number of proposals for URN schemes that create otherwise
   unbound "names", where the URN scheme only provides for uniqueness.
   Neither "duri" nor "tdb" intrinsically have the property that the
   names assigned are without any resolution semantics.  This is
   intentional; it's difficult to create names that carry no semantics
   whatsoever about the authority that assigned the name and the
   intention of the authority for what the name should designate.

5.7.  Avoiding MetaData

   One might consider the date in a duri/tdb to be just one piece of
   additional metadata about the encoded-URI, and consider adding other
   pieces of metadata as annotation.

   However, the use of the date in a duri/tdb is intended primarily as a
   mechanism of accomplishing uniqueness over time.  No other bit of
   metadata or description readily fills that purpose.  Further, the
   date is not descriptive (an assertion about the encoded-URI) but
   merely refining.

5.8.  Avoiding duri and tdb

   Many applications of URIs already provide a context of date.  For
   example, one could imagine a hypertext system where the URIs
   contained within a document were intended to refer to the resources
   as of the date of the enclosing document.  This would be a reasonable
   interpretation of URIs within an Internet archive system, for

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   And some applications of URIs arguably already contain the level of
   interpretive indirection that is explicit with "tdb".  For example,
   one might consider the use of URIs as namespace names within XML [5]
   as a reference to the "thing described by" the URI used.

5.9.  tdb and levels of indirection

   The "tdb" scheme introduces a level of semantic indirection.  The
   puzzles and confusions about use and mention, name and reference, and
   levels of indirection have been puzzling and amusing for quite a
      "It's long," said the Knight, "but it's very, very beautiful.
      Everybody that hears me sing it--either it brings tears into their
      eyes, or else--"
      "Or else what?" said Alice, for the Knight had made a sudden
      "Or else it doesn't, you know.  The name of the song is called
      'Haddock's Eyes.'"
      "Oh, that's the name of the song, is it?"  Alice said, trying to
      feel interested.
      "No, you don't understand," the knight said, looking a little
      vexed.  "That's what the name is called.  The name really is 'The
      Aged Aged Man.'"
      "Then I ought to have said 'That's what the song is called'?"
      Alice corrected herself.
      "No, you oughtn't: that's quite another thing!  The song is called
      'Ways and Means': but that's only what it's called, you know!"
      "Well, what is the song, then?" said Alice, who was by this time
      completely bewildered.
      "I was coming to that," the Knight said.  "The song really is
      'A-sitting On A Gate': and the tune's my own invention." [10]

6.  URN Specification Templates

6.1.  duri Specification Template

   Namespace ID:  "duri" requested.
   Registration Information:  Registration Version: 1
      Registration Date: 2001-08-19
   Declared registrant of the namespace:  Larry Masinter
   Declaration of syntactic structure:  Briefly, the syntax is
      The syntax is described in this document.

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   Relevant ancillary documentation:  (See References of this document)
   Identifier uniqueness considerations:  Uniqueness is guaranteed by
      the structure of adding a designation of a specific instant to a
      URI.  However, URIs with ambiguous interpretation at any given
      instant (e.g., "file" URIs without a given host name) will not be
   Identifier persistence considerations:  The designation of a dated
      URI is completely persistent for all time.
   Process of identifier assignment:  Any date can be used with any URI
      independently by anyone.
   Process of identifier resolution:  Identifiers can only be resolved
      approximately.  See Section 5.5.
   Conformance with URN Syntax:  Note that the use of "/" for hierarchy,
      while discouraged in the URN specification, is allowed in duris.
   Rules for Lexical Equivalent:  For dates, YYYY is equivalent to
      YYYY01, YYYYMM is equivalent to YYYYMM01, while YYYYMMDD is
      equivalent to YYYYMMDD0... followed by any number of 0's.  In
      considering equivalence of the encoded URI, if two duris with
      equivalent dates contain lexically equivalent URIs, the duris are
   Validation mechanism:  Dates should be reasonable and meet the
      syntactic requirements.  The URI encoded within should meet the
      syntactic requirements of the URI scheme used.
   Scope:  Global.

6.2.  tdb Specification Template

   Namespace ID:  "tdb" requested.
   Registration Information:  Registration Version: 1
      Registration Date: 2002-04-01
   Declared registrant of the namespace:  Larry Masinter
   Declaration of syntactic structure:  Briefly, the syntax is
      The syntax is described in Section 2.1.
   Relevant ancillary documentation:  (See References of this document)
   Identifier uniqueness considerations:  Uniqueness is guaranteed by
      the structure of adding a designation of a specific instant to a
      URI.  However, URIs with ambiguous interpretation at any given
      instant (e.g., "file" URIs without a given host name) will not be
   Identifier persistence considerations:  The designation of a dated
      URI is completely persistent for all time, although the intent of
      a resource that is no longer available will be hard to discern.
   Process of identifier assignment:  Any date can be used with any URI
      independently by anyone.  However, assigning an identifier to a
      non-networked resource such as a person or abstract concept
      requires (at least conceptually) first creating a networked
      resource that uniquely describes the target, and then constructing

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      the duri using the URI of the description.
   Process of identifier resolution:  Resolution of "tdb" identifiers
      requires interpreting the resource identified by the corresponding
      "duri".  See Section 2.2 of this document.
   Rules for Lexical Equivalent:  As with "duri"; see Section 6.1.
   Conformance with URN Syntax:  As with "duri"; see Section 6.1.
   Validation mechanism:  As with "duri", see Section 6.1.
   Scope:  Global.

7.  IANA considerations

   This document includes two URN NID registrations (Section 6.1 and
   Section 6.2) that should be entered into the IANA registry of URN

8.  Security Considerations

   "duri" and "tdb" identifiers are not any more reliable because they
   have dates.  URIs don't contain enough information to supply the
   authority for deciding what was or wasn't at a given URI at a given

9.  Acknowledgements

   There have been many discussions over several years on the
   relationship of URLs, URNs, URIs, resources and resource identifiers,
   with many contributions.  Particular thanks to Aaron Swartz, Brian
   McBride and Stuart Williams, Michael Mealling, and many others.

10.  References

10.1.  Normative References

   [1]   Moats, R., "URN Syntax", RFC 2141, May 1997.

   [2]   Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
         August 1999.

   [3]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 3986,
         January 2005.

   [4]   Bureau International des Poids et Mesures, "International
         Atomic Time".

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Internet-Draft         duri and tdb URN namespaces         February 2009

   [5]   Bray, T., Hollander, D., and A. Layman, "Namespaces in XML",
         W3C Recommendation REC-xml-names, January 1999,

10.2.  Informative References

   [6]   Berners-Lee, T., "Cool URIs don't change", 1998,

   [7]   Sollins, K., "Functional Requirements for Uniform Resource
         Names", RFC 1737, December 1994.

   [8]   Glassman, S., Manasse, M., and J. Mogul, "Y10K and Beyond",
         RFC 2550, April 1 1999.

   [9]   Kahle, B., "Preserving the Internet", Scientific American ,
         March 1997, <>.

   [10]  Carroll, L., "Through the Looking Glass", 1872, <http://

Author's Address

   Larry Masinter
   Adobe Systems
   345 Park Ave
   San Jose, CA  95110

   Phone: +1 408 536 3024

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