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166 lines
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>23.1.2. Case Conventions</TITLE>
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<meta name="description" value=" Case Conventions">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html4119 HREF="node206.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4117 HREF="node203.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4111 HREF="node204.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4121 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4122 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4120 HREF="node206.html"> Structured Directories</A>
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<B>Up:</B> <A NAME=tex2html4118 HREF="node203.html"> File Names</A>
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<B> Previous:</B> <A NAME=tex2html4112 HREF="node204.html"> Pathnames</A>
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<HR> <P>
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<H2><A NAME=SECTION002712000000000000000>23.1.2. Case Conventions</A></H2>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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<A NAME=PATHNAMECOMPONENTCASESECTION>Issues</A>
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of alphabetic case in pathnames are a major source of problems.
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In some file systems, the customary case is lowercase, in some uppercase,
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in some mixed. Some file systems are case-sensitive (that is, they treat
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<tt>FOO</tt> and <tt>foo</tt> as different file names) and others are not.
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<P>
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There are two kinds of pathname case portability problems: moving
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programs from one Common Lisp to another, and moving pathname component
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values from one file system to another. The solution to the first problem
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is the requirement that all
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Common Lisp implementations that support a particular file system must
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use compatible representations for pathname component values. The solution to
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the second problem is the use of a common representation for the
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least-common-denominator pathname component values that exist on all
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interesting file systems.
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<P>
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Requiring a common representation directly conflicts with the
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desire among programmers that use only one file system to work with the
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local conventions and to ignore issues of porting to other file
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systems. The common representation cannot be the same as local (varying)
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conventions.
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<P>
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X3J13 voted in June 1989 (PATHNAME-COMPONENT-CASE) <A NAME=26133> </A> to
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add a keyword argument <tt>:case</tt> to each of the functions
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<tt>make-pathname</tt>, <tt>pathname-host</tt>,
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<tt>pathname-device</tt>, <tt>pathname-directory</tt>, <tt>pathname-name</tt>,
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and <tt>pathname-type</tt>.
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The possible values for the argument are <tt>:common</tt> and <tt>:local</tt>.
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The default is <tt>:local</tt>.
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<P>
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The value <tt>:local</tt> means that strings given to <tt>make-pathname</tt>
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or returned by any of the pathname component accessors
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follow the local file system's conventions for alphabetic case.
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Strings given to <tt>make-pathname</tt> will be used exactly as written if
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the file system supports both cases. If the file system
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supports only one case, the strings will be translated to that case.
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<P>
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The value <tt>:common</tt> means that strings given to <tt>make-pathname</tt>
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or returned by any of the pathname component accessors
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follow this common convention:
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<UL><LI> All uppercase means that a file system's customary case will be used.
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<LI> All lowercase means that the opposite of the customary case will be used.
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<LI> Mixed case represents itself.
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</UL>
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Uppercase is used as the common case for no better reason than
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consistency with Lisp symbols.
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The second and third points allow translation from local representation to
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common and back to be information-preserving. (Note that translation
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from common to local representation and back may or may not be information-preserving,
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depending on the nature of the local representation.)
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<P>
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Namestrings always use <tt>:local</tt> file system case conventions.
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<P>
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Finally, <tt>merge-pathnames</tt> and <tt>translate-pathname</tt> map customary case in the
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input pathnames into customary case in the output pathname.
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<P>
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Examples of possible use of this convention:
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<UL> <LI> TOPS-20 is case-sensitive and prefers uppercase,
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translating lowercase to uppercase unless escaped with <tt>^V</tt>;
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for a TOPS-20-based
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file system, a Common Lisp implementation
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should use identical
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representations for common and local.
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<P>
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<LI> UNIX is case-sensitive and prefers lowercase; for a UNIX-based file system,
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a Common Lisp implementation should translate between
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common and local representations by inverting the case of non-mixed-case strings.
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<P>
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<LI> VAX/VMS is uppercase-only (that is, the file system translates all file
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name arguments to uppercase); for a VAX/VMS-based file system,
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a Common Lisp implementation should
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translate common representation to local by
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converting to uppercase and should translate local representation
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to common with no change.
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<P>
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<LI> The Macintosh operating system is case-insensitive and prefers lowercase,
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but remembers the cases of letters actually used to name a file;
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for a Macintosh-based file system, a Common Lisp implementation should translate
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between common and local representations by inverting the case of non-mixed-case strings
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and should ignore case when determining whether two pathnames are <tt>equal</tt>.
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</UL>
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<P>
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Here are some examples of this behavior. Assume that the host <tt>T</tt> runs
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TOPS-20, <tt>U</tt> runs UNIX, <tt>V</tt> runs VAX/VMS, and <tt>M</tt> runs the Macintosh
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operating system.
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<P><pre>
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;;; Returns two values: the PATHNAME-NAME from a namestring
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;;; in :COMMON and :LOCAL representations (in that order).
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(defun pathname-example (name)
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(let ((path (parse-namestring name))))
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(values (pathname-name path :case :common)
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(pathname-name path :case :local))))
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=> "FOO" and ¯"FOO" ;Common Local
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(pathname-example "T:<ME>FOO.LISP") => "FOO" and "FOO"
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(pathname-example "T:<ME>foo.LISP") => "FOO" and "FOO"
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(pathname-example "T:<ME>^Vf^Vo^Vo.LISP") => "foo" and "foo"
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(pathname-example "T:<ME>TeX.LISP") => "TEX" and "TEX"
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(pathname-example "T:<ME>T^VeX.LISP") => "TeX" and "TeX"
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(pathname-example "U:/me/FOO.lisp") => "foo" and "FOO"
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(pathname-example "U:/me/foo.lisp") => "FOO" and "foo"
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(pathname-example "U:/me/TeX.lisp") => "TeX" and "TeX"
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(pathname-example "V:[me]FOO.LISP") => "FOO" and "FOO"
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(pathname-example "V:[me]foo.LISP") => "FOO" and "FOO"
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(pathname-example "V:[me]TeX.LISP") => "TEX" and "TEX"
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(pathname-example "M:FOO.LISP") => "foo" and "FOO"
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(pathname-example "M:foo.LISP") => "FOO" and "foo"
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(pathname-example "M:TeX.LISP") => "TeX" and "TeX"
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</pre><P>
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The following example illustrates the creation of new pathnames.
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The name is converted from common representation to local because
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namestrings always use local conventions.
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<P><pre>
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(defun make-pathname-example (h n)
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(namestring (make-pathname :host h :name n :case :common))
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(make-pathname-example "T" "FOO") => "T:FOO"
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(make-pathname-example "T" "foo") => "T:^Vf^Vo^Vo"
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(make-pathname-example "T" "TeX") => "T:T^VeX"
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(make-pathname-example "U" "FOO") => "U:foo"
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(make-pathname-example "U" "foo") => "U:FOO"
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(make-pathname-example "U" "TeX") => "U:TeX"
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(make-pathname-example "V" "FOO") => "V:FOO"
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(make-pathname-example "V" "foo") => "V:FOO"
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(make-pathname-example "V" "TeX") => "V:TeX"
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(make-pathname-example "M" "FOO") => "M:foo"
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(make-pathname-example "M" "foo") => "M:FOO"
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(make-pathname-example "M" "TeX") => "M:TeX"
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</pre><P>
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A big advantage of this set of conventions is that one can, for example,
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call <tt>make-pathname</tt> with <tt>:type "LISP"</tt> and <tt>:case :common</tt>,
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and the result will appear in a namestring as <tt>.LISP</tt> or <tt>.lisp</tt>,
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whichever is appropriate.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html4119 HREF="node206.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4117 HREF="node203.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4111 HREF="node204.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4121 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4122 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4120 HREF="node206.html"> Structured Directories</A>
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<B>Up:</B> <A NAME=tex2html4118 HREF="node203.html"> File Names</A>
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<B> Previous:</B> <A NAME=tex2html4112 HREF="node204.html"> Pathnames</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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