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Versions: (draft-masinter-iri-comparison) 00 01 02

Internationalized Resource Identifiers                       L. Masinter
(iri)                                                              Adobe
Internet-Draft                                                 M. Duerst
Updates: 3986 (if approved)                     Aoyama Gakuin University
Intended status: Standards Track                        October 23, 2012
Expires: April 26, 2013

   Comparison, Equivalence and Canonicalization of Internationalized
                          Resource Identifiers


   Internationalized Resource Identifiers (IRIs) are Unicode strings
   used to identify resources on the Internet.  Applications that use
   IRIs often define a means of comparing IRIs to determine when two
   IRIs are equivalent for the purpose of that application.  Some
   applications also define a method for canonicalizing an IRI --
   translating one IRI into another which is equivalent under the
   comparison method used.

   This document gives guidelines and best practices for defining and
   using IRI comparison and canonicalization methods.

   Comparison methods are used to determine equivalence.  As URIs are a
   subset of IRIs, the guidelines apply to URI comparison as well.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on April 26, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  General guidelines . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Preparation for Comparison . . . . . . . . . . . . . . . . . .  5
   4.  Comparison Hierarchy . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Simple String Comparison . . . . . . . . . . . . . . . . .  6
     4.2.  Syntax-Based Equivalence . . . . . . . . . . . . . . . . .  7
       4.2.1.  Case Equivalence . . . . . . . . . . . . . . . . . . .  8
       4.2.2.  Unicode Character Normalization  . . . . . . . . . . .  8
       4.2.3.  Percent-Encoding Equivalence . . . . . . . . . . . . .  9
       4.2.4.  Path Segment Equivalence . . . . . . . . . . . . . . . 10
     4.3.  Scheme-Based Comparison  . . . . . . . . . . . . . . . . . 10
     4.4.  Protocol-Based Comparison  . . . . . . . . . . . . . . . . 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

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1.  Introduction

   Internationalized Resource Identifiers (IRIs) are Unicode strings
   used to identify resources on the Internet.  Applications that use
   IRIs often define a means of comparing IRIs to determine when two
   IRIs are equivalent for the purpose of that application.  Some
   applications also define a method for canonicalizing an IRI --
   translating one IRI into another which is equivalent under the
   comparison method used.

   This document gives guidelines and best practices for defining and
   using IRI comparison and canonicalization methods.

   As every URI is also an IRI, the comparison and canonicalization
   methods also apply to URIs.

   IRI comparison is expected to determine whether two IRIs are
   equivalent without using the IRIs to access their respective
   resource(s).  For example, comparisons are performed whenever a
   response cache is accessed, a browser checks its history to color a
   link, or an XML parser processes tags within a namespace.

   Comparison for equivalence is often accomplished by canonicalization:
   (sometimes called normalization): a process for converting data that
   has more than one possible representation into a "standard",
   "normal", or "canonical" form.  Extensive canonicalization prior to
   comparison of IRIs may be used by spiders and indexing engines to
   prune a search space or reduce duplication of request actions and
   response storage.

   IRI comparison is performed for some particular purpose.  Protocols
   or implementations that compare IRIs for different purposes will
   often be subject to differing design trade-offs in regards to how
   much effort should be spent in reducing aliased identifiers.  This
   document describes various methods that may be used to compare IRIs,
   the trade-offs between them, and the types of applications that might
   use them.

2.  General guidelines

   Because IRIs exist to identify resources, one might expect two IRIs
   to be considered equivalent when they identify the same resource.
   However, this definition of equivalence is not of much practical use,
   as there is in general no way for an implementation to compare two
   resources to determine if they are "the same" unless it has full
   knowledge or control of them.  Comparison methods for IRIs are
   generally based strictly on examining the characters that make up the

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   IRI, without performing any network access.

   We use the terms "different" and "equivalent" to describe the
   possible outcomes of such comparisons, but there are many
   application-dependent versions of equivalence.

   Even when it is possible to determine that two IRIs are equivalent,
   IRI comparison is not sufficient to determine whether two IRIs
   identify different resources.  For example, an owner of two different
   domain names could decide to serve the same resource from both,
   resulting in two different IRIs.  For this reason, false negatives
   (e.g., returning "different" even with the resources are "the same")
   cannot be completely avoided.  Comparison methods often try to
   minimize false negatives while strictly avoiding false positives.
   However, in some cases (such as cache invalidation), false negatives
   are more harmful than false positives.

   A comparison method for determining equivalence might have multiple
   values, for example, returning "equivalent", "different", or
   "equivalence cannot be determined".

   Multiple canonicalization (normalizations) methods might be defined,
   where sequential application of each results in greater sets of
   equivalent values.

   In testing for equivalence, applications should not directly compare
   relative references; the references should be converted to their
   respective target IRIs before comparison. [[ref 3987bis]]

   Some IRIs contain fragment identifiers.  In general, the equivalence
   of two IRIs is determined first by comparing the IRIs without any
   fragment identifiers, and then (if appropriate) the fragment
   components (if any) compared.

   Some applications (such as XML namespaces) use IRIs as identity
   tokens without any relationship to acessing the resources.  Those
   applications use the Simple String Comparison (see Section 4.1).

3.  Preparation for Comparison

   Any kind of IRI comparison REQUIRES that any additional contextual
   processing is first performed, including undoing higher-level
   escapings or encodings in the protocol or format that carries an IRI.
   This preprocessing is usually done when the protocol or format is

   NOTE: This document has not yet been updated to use in-line Unicode

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   Examples of such escapings or encodings are entities and numeric
   character references in [HTML4] and [XML1].  As an example,
   "http://example.org/rosé" (in HTML),
   "http://example.org/rosé" (in HTML or XML), and
   "http://example.org/rosé" (in HTML or XML) are all resolved into
   what is denoted in this document (see 'Notation' section of
   [RFC3987bis]) as "http://example.org/rosé" (the "é" here
   standing for the actual e-acute character, to compensate for the fact
   that this document cannot contain non-ASCII characters).

   An IRI is a sequence of Unicode characters.  IRIs are sometimes
   represented in documents as sequences of bytes in a charset, either
   Unicode-based (UTF-8) or using some other character encoding (e.g.,
   ISO-8859-1).  Before comparing two such sequences, they must both be
   converted into sequences of Unicode characters.

   Similarly, encodings such as Transfer Codings in HTTP (see [RFC2616])
   and Content Transfer Encodings in MIME ([RFC2045]) must be unencoded.
   In these cases, the encoding is based not on characters but on
   octets, and additional care is required to make sure that characters,
   and not just arbitrary octets, are compared (see Section 4.1.

4.  Comparison Hierarchy

   In practice, a variety of methods are used to test IRI equivalence.
   These methods generally fall into a range distinguished by the amount
   of processing required and the degree to which the probability of
   false negatives is reduced.  As noted above, false negatives cannot
   be eliminated.  In practice, their probability can be reduced, but
   this reduction requires more processing and is not cost-effective for
   all applications.

   The following discussion starts with comparison methods that are
   cheap but have a relatively higher chance of producing false
   negatives, and proceeding to those that have higher computational
   cost and lower risk of false negatives.

4.1.  Simple String Comparison

   If two IRIs (when considered as strings of Unicode characters) are
   identical, then it is safe to conclude that they are equivalent.
   This type of equivalence test has very low computational cost and is
   in wide use in a variety of applications, particularly in the domain
   of parsing.  It is also used when a definitive answer to the question
   of IRI equivalence is needed that is independent of the scheme used

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   and that can be calculated quickly and without accessing a network.
   An example of such a case is XML Namespaces ([XMLNamespace]).

   Testing strings for equivalence requires some basic precautions.
   This procedure is often referred to as "bit-for-bit" or "byte-for-
   byte" comparison, which is potentially misleading.  Testing strings
   for equality is normally based on pair comparison of the characters
   that make up the strings, starting from the first and proceeding
   until both strings are exhausted and all characters are found to be
   equal, until a pair of characters compares unequal, or until one of
   the strings is exhausted before the other.

   This character comparison requires that each pair of characters be
   put in comparable encoding form.  For example, should one IRI be
   stored in a byte array in UTF-8 encoding form and the second in a
   UTF-16 encoding form, bit-for-bit comparisons applied naively will
   produce errors.  It is better to speak of equality on a character-
   for-character rather than on a byte-for-byte or bit-for-bit basis.
   In practical terms, character-by-character comparisons should be done
   codepoint by codepoint after conversion to a common character
   encoding form.  When comparing character by character, the comparison
   function MUST NOT map IRIs to URIs, because such a mapping would
   create additional spurious equivalences.  It follows that an IRI
   SHOULD NOT be modified when being transported if there is any chance
   that this IRI might be used in a context that uses Simple String

   False negatives are caused by the production and use of IRI aliases.
   Unnecessary aliases can be reduced, regardless of the comparison
   method, by consistently providing IRI references in a canonical form
   (after canonicalization is applied).

   Protocols and data formats might limit some IRI comparisons to simple
   string comparison, based on the theory that people and
   implementations will, in their own best interest, be consistent in
   providing IRI references, or at least be consistent enough to negate
   any efficiency that might be obtained from further canonicalization.

4.2.  Syntax-Based Equivalence

   Implementations may use logic based on the definitions provided by
   this specification to reduce the probability of false negatives.
   This processing is moderately higher in cost than character-for-
   character string comparison.  For example, an application using this
   approach could reasonably consider the following two IRIs equivalent:


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   Web user agents, such as browsers, typically apply this type of IRI
   equivalence when determining whether a cached response is available.
   Syntax-based equivalence includes such techniques as case
   equivalence, Unicode character normalization, percent-encoding
   equivalence, and removal of dot-segments.

4.2.1.  Case Equivalence

   For all IRIs, the hexadecimal digits within a percent-encoding
   triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
   should be considered equivalent to forms which use uppercase letters
   for the digits A-F.

   When an IRI uses components of the generic syntax, the component
   syntax equivalence rules always apply; namely, that the scheme and
   US-ASCII only host are case insensitive and therefore should be
   treated equivalent to lowercase.  For example, the URI
   "HTTP://www.EXAMPLE.com/" is equivalent to "http://www.example.com/".
   Case equivalence for non-ASCII characters in IRI components that are
   IDNs are discussed in Section 4.3.  The other generic syntax
   components are assumed to be case sensitive unless specifically
   defined otherwise by the scheme.

   Creating schemes that allow case-insensitive syntax components
   containing non-ASCII characters should be avoided.  Case equivalence
   of non-ASCII characters can be culturally dependent and is always a
   complex operation.  The only exception concerns non-ASCII host names
   for which the character normalization includes a mapping step derived
   from case folding.

4.2.2.  Unicode Character Normalization

   The Unicode Standard [UNIV6] defines various equivalences between
   sequences of characters for various purposes.  Unicode Standard Annex
   #15 [UTR15] defines various Normalization Forms for these
   equivalences, in particular Normalization Form C (NFC, Canonical
   Decomposition, followed by Canonical Composition) and Normalization
   Form KC (NFKC, Compatibility Decomposition, followed by Canonical

   IRIs already in Unicode MUST NOT be normalized before parsing or
   interpreting.  In many non-Unicode character encodings, some text
   cannot be represented directly.  For example, the word "Vietnam" is
   natively written "Việt Nam" (containing a LATIN SMALL LETTER E
   WITH CIRCUMFLEX AND DOT BELOW) in NFC, but a direct transcoding from
   the windows-1258 character encoding leads to "Việt Nam"
   (containing a LATIN SMALL LETTER E WITH CIRCUMFLEX followed by a
   COMBINING DOT BELOW).  Direct transcoding of other 8-bit encodings of

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   Vietnamese may lead to other representations.

   Equivalence of IRIs MUST rely on the assumption that IRIs are
   appropriately pre-character-normalized rather than apply character
   normalization when comparing two IRIs.  The exceptions are conversion
   from a non-digital form, and conversion from a non-UCS-based
   character encoding to a UCS-based character encoding.  In these
   cases, NFC or a normalizing transcoder using NFC MUST be used for
   interoperability.  To avoid false negatives and problems with
   transcoding, IRIs SHOULD be created by using NFC.  Using NFKC may
   avoid even more problems; for example, by choosing half-width Latin
   letters instead of full-width ones, and full-width instead of half-
   width Katakana.

   As an example, "http://www.example.org/résumé.html" (in XML
   Notation) is in NFC.  On the other hand,
   "http://www.example.org/résumé.html" is not in NFC.

   The former uses precombined e-acute characters, and the latter uses
   "e" characters followed by combining acute accents.  Both usages are
   defined as canonically equivalent in [UNIV6].

   Note:  Because it is unknown how a particular sequence of characters
      is being treated with respect to character normalization, it would
      be inappropriate to allow third parties to normalize an IRI
      arbitrarily.  This does not contradict the recommendation that
      when a resource is created, its IRI should be as character
      normalized as possible (i.e., NFC or even NFKC).  This is similar
      to the uppercase/lowercase problems.  Some parts of a URI are case
      insensitive (for example, the domain name).  For others, it is
      unclear whether they are case sensitive, case insensitive, or
      something in between (e.g., case sensitive, but with a multiple
      choice selection if the wrong case is used, instead of a direct
      negative result).  The best recipe is that the creator use a
      reasonable capitalization and, when transferring the URI,
      capitalization never be changed.

   Various IRI schemes may allow the usage of Internationalized Domain
   Names (IDN) [RFC5890] either in the ireg-name part or elsewhere.
   Character Normalization also applies to IDNs, as discussed in
   Section 4.3.

4.2.3.  Percent-Encoding Equivalence

   The percent-encoding mechanism (Section 2.1 of [RFC3986]) is a
   frequent source of variance among otherwise identical IRIs.  In
   addition to the case equivalence issue noted above, some IRI
   producers percent-encode octets that do not require percent-encoding,

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   resulting in IRIs that are equivalent to their nonencoded
   counterparts.  These IRIs should be compared by first decoding any
   percent-encoded octet sequence that corresponds to an unreserved
   character, as described in section 2.3 of [RFC3986].

   For actual resolution, differences in percent-encoding (except for
   the percent-encoding of reserved characters) SHOULD always result in
   the same resource.  For example, "http://example.org/~user",
   "http://example.org/%7euser", and "http://example.org/%7Euser",
   SHOULD resolve to the same resource.

   If this kind of equivalence is to be tested, the percent-encoding of
   both IRIs to be compared first needs to be aligned; for example, by
   converting both IRIs to URIs, eliminating escape differences in the
   resulting URIs, and making sure that the case of the hexadecimal
   characters in the percent-encoding is always the same (preferably
   upper case).  If the IRI is to be passed to another application or
   used further in some other way, its original form MUST be preserved.
   The conversion described here should be performed only for local

4.2.4.  Path Segment Equivalence

   The complete path segments "." and ".." are intended only for use
   within relative references (Section 4.1 of [RFC3986]) and are removed
   as part of the reference resolution process (Section 5.2 of
   [RFC3986]).  However, some implementations may incorrectly assume
   that reference resolution is not necessary when the reference is
   already an IRI, and thus fail to remove dot-segments when they occur
   in non-relative paths.  IRI comparison SHOULD remove dot-segments by
   applying the remove_dot_segments algorithm to the path, as described
   in Section 5.2.4 of [RFC3986].

4.3.  Scheme-Based Comparison

   The syntax and semantics of IRIs vary from scheme to scheme, as
   described by the defining specification for each scheme.
   Implementations may use scheme-specific rules, at further processing
   cost, to reduce the probability of false negatives.  For example,
   because the "http" scheme makes use of an authority component, has a
   default port of "80", and defines an empty path to be equivalent to
   "/", the following four IRIs are equivalent:


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   In general, an IRI that uses the generic syntax for authority with an
   empty path should be equivalent to a path of "/".  Likewise, an
   explicit ":port", for which the port is empty or the default for the
   scheme, is equivalent to one where the port and its ":" delimiter are

   Another case where equivalence varies by scheme is in the handling of
   an empty authority component or empty host subcomponent.  For many
   scheme specifications, an empty authority or host is considered an
   error; for others, it is considered equivalent to "localhost" or the
   end-user's host.

   The presence of a missing component vs. one with an empty string
   component in an IRI SHOULD NOT be treated as equivalent unless
   explicitly defined as such by the scheme definition.  For example,
   the IRI "http://example.com/?" cannot be assumed to be equivalent to
   any of the examples above; an empty query component is NOT equivalent
   to a missing one.  Likewise, the presence or absence of delimiters
   within a userinfo subcomponent is usually significant to its
   interpretation.  The fragment component is not subject to any scheme-
   based equivalence; thus, two IRIs that differ only by the suffix "#"
   are considered different regardless of the scheme.

   Some IRI schemes allow the usage of Internationalized Domain Names
   (IDN) [RFC5890] either in their ireg-name part or elswhere.  When in
   use in IRIs, those names SHOULD conform to the definition of U-Label
   in [RFC5890].  An IRI containing an invalid IDN cannot successfully
   be resolved.  For legibility purposes, they SHOULD NOT be converted
   into ASCII Compatible Encoding (ACE).

   Scheme-based comparison may also consider IDN components and their
   conversions to punycode as equivalent.  As an example,
   "http://résumé.example.org" may be considered equivalent to

   Other scheme-specific equivalence rules are possible.

4.4.  Protocol-Based Comparison

   Substantial effort to reduce the incidence of false negatives is
   often cost-effective for web spiders.  Consequently, they implement
   even more aggressive techniques in IRI comparison.  For example, if
   they observe that an IRI such as


   redirects to an IRI differing only in the trailing slash

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   they will likely regard the two as equivalent in the future.  This
   kind of technique is only appropriate when equivalence is clearly
   indicated by both the result of accessing the resources and the
   common conventions of their scheme's dereference algorithm (in this
   case, use of redirection by HTTP origin servers to avoid problems
   with relative references).

5.  Security Considerations

   The primary security difficulty comes from applications choosing the
   wrong equivalence relationship, or two different parties disagreeing
   on equivalence.  This is especially a problem when IRIs are used in
   security protocols.

   Besides the large character repertoire of Unicode, reasons for
   confusion include different forms of normalization and different
   normalization expectations, use of percent-encoding with various
   legacy encodings, and bidirectionality issues.  See also [UTR36].

6.  Acknowledgements

   This document was originally derived from [RFC3986] and [RFC3987],
   based on text contributed by Tim Bray.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, March 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

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              Duerst, M., Masinter, L., and M. Suignard,
              "Internationalized Resource Identifiers (IRIs)", 2012,

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [UNIV6]    The Unicode Consortium, "The Unicode Standard, Version
              6.0.0 (Mountain View, CA, The Unicode Consortium, 2011,
              ISBN 978-1-936213-01-6)", October 2010.

   [UTR15]    Davis, M. and M. Duerst, "Unicode Normalization Forms",
              Unicode Standard Annex #15, March 2008,

7.2.  Informative References

   [HTML4]    Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation,
              December 1999,

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [UTR36]    Davis, M. and M. Suignard, "Unicode Security
              Considerations", Unicode Technical Report #36,
              August 2010, <http://unicode.org/reports/tr36/>.

   [XML1]     Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Forth
              Edition)", World Wide Web Consortium Recommendation,
              August 2006, <http://www.w3.org/TR/REC-xml>.

              Bray, T., Hollander, D., Layman, A., and R. Tobin,
              "Namespaces in XML (Second Edition)", World Wide Web

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              Consortium Recommendation, August 2006,

Authors' Addresses

   Larry Masinter
   345 Park Ave
   San Jose, CA  95110

   Phone: +1-408-536-3024
   Email: masinter@adobe.com
   URI:   http://larry.masinter.net

   Martin Duerst
   Aoyama Gakuin University
   5-10-1 Fuchinobe
   Sagamihara, Kanagawa  229-8558

   Phone: +81 42 759 6329
   Fax:   +81 42 759 6495
   Email: duerst@it.aoyama.ac.jp
   URI:   http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/

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