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162 lines
8.3 KiB
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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>29.4.3. Exhaustive Case Analysis</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Exhaustive Case Analysis">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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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=tex2html5908 HREF="node338.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5906 HREF="node334.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5900 HREF="node336.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5910 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5911 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5909 HREF="node338.html"> Handling Conditions</A>
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<B>Up:</B> <A NAME=tex2html5907 HREF="node334.html"> Program Interface to </A>
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<B> Previous:</B> <A NAME=tex2html5901 HREF="node336.html"> Assertions</A>
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<HR> <P>
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<H2><A NAME=SECTION003343000000000000000>29.4.3. Exhaustive Case Analysis</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=EXHAUSTIVECASEANALYSISCONDITIONS>The</A>
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syntax for <tt>etypecase</tt> and <tt>ctypecase</tt> is the same as for <tt>typecase</tt>, except
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that no <tt>otherwise</tt> clause is permitted. Similarly, the syntax for <tt>ecase</tt> and
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<tt>ccase</tt> is the same as for <tt>case</tt> except for the <tt>otherwise</tt> clause.
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<P>
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<tt>etypecase</tt> and <tt>ecase</tt> are similar to <tt>typecase</tt> and <tt>case</tt>, respectively, but signal
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a non-continuable error rather than returning <tt>nil</tt> if no clause is selected.
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<P>
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<tt>ctypecase</tt> and <tt>ccase</tt> are also similar to <tt>typecase</tt> and <tt>case</tt>, respectively,
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but signal a
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continuable error if no clause is selected.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>etypecase <i>keyform</i> {(<i>type</i> {<i>form</i>}*)}*</tt><P> [This supersedes the description of <tt>etypecase</tt>
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given in section <A HREF="node222.html#EXHAUSTIVECASEANALYSIS">24.3</A>.-GLS]
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<P>
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This control construct is similar to <tt>typecase</tt>, but no explicit <tt>otherwise</tt>
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or <tt>t</tt> clause is permitted. If no clause is satisfied, <tt>etypecase</tt> signals
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an error (of type <tt>type-error</tt>) with a message constructed from the clauses.
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It is not permissible to continue from this error. To supply an error
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message, the user should use <tt>typecase</tt> with an <tt>otherwise</tt> clause containing
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a call to <tt>error</tt>. The name of this function stands for ``exhaustive type
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case'' or ``error-checking type case.''
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<P>
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Example:
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<P><pre>
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Lisp> (setq x 1/3)
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=> 1/3
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Lisp> (etypecase x
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(integer (* x 4))
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(symbol (symbol-value x)))
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Error: The value of X, 1/3, is neither an integer nor a symbol.
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To continue, type :CONTINUE followed by an option number:
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1: Return to Lisp Toplevel.
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Debug>
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</pre><P>
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>ctypecase <i>keyplace</i> {(<i>type</i> {<i>form</i>}*)}*</tt><P> [This supersedes the description of <tt>ctypecase</tt>
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given in section <A HREF="node222.html#EXHAUSTIVECASEANALYSIS">24.3</A>.-GLS]
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<P>
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This control construct is similar to <tt>typecase</tt>, but no explicit
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<tt>otherwise</tt> or <tt>t</tt> clause is permitted.
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<P>
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The <i>keyplace</i> must be a generalized variable reference acceptable to <tt>setf</tt>.
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If no clause is satisfied, <tt>ctypecase</tt> signals an error (of type <tt>type-error</tt>)
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with a message constructed from the clauses. This error may be continued
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using the <tt>store-value</tt> restart. The argument to <tt>store-value</tt> is stored in
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<i>keyplace</i> and then <tt>ctypecase</tt> starts over, making the type tests again.
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Subforms of <i>keyplace</i> may be evaluated multiple times. If the <tt>store-value</tt>
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restart is invoked interactively, the user will be prompted for the value
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to be used.
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<P>
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The name of this function is mnemonic for ``continuable (exhaustive)
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type case.''
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<P>
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Example:
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<P><pre>
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Lisp> (setq x 1/3)
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=> 1/3
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Lisp> (ctypecase x
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(integer (* x 4))
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(symbol (symbol-value x)))
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Error: The value of X, 1/3, is neither an integer nor a symbol.
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To continue, type :CONTINUE followed by an option number:
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1: Specify a value to use instead.
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2: Return to Lisp Toplevel.
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Debug> :continue 1
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Use value: 3.7
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Error: The value of X, 3.7, is neither an integer nor a symbol.
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To continue, type :CONTINUE followed by an option number:
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1: Specify a value to use instead.
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2: Return to Lisp Toplevel.
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Debug> :continue 1
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Use value: 12
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=> 48
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</pre><P>
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>ecase <i>keyform</i> {({({<i>key</i>}*) | <i>key</i>} {<i>form</i>}*)}*</tt><P> [This supersedes the description of <tt>ecase</tt>
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given in section <A HREF="node222.html#EXHAUSTIVECASEANALYSIS">24.3</A>.-GLS]
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<P>
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This control construct is similar to <tt>case</tt>, but no explicit <tt>otherwise</tt> or <tt>t</tt>
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clause is permitted. If no clause is satisfied, <tt>ecase</tt> signals an error
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(of type <tt>type-error</tt>) with a message constructed from the clauses. It is not
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permissible to continue from this error. To supply an error message, the
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user should use <tt>case</tt> with an <tt>otherwise</tt> clause containing a call to <tt>error</tt>.
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The name of this function stands for ``exhaustive case'' or ``error-checking
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case.''
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<P>
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Example:
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<P><pre>
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Lisp> (setq x 1/3)
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=> 1/3
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Lisp> (ecase x
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(alpha (foo))
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(omega (bar))
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((zeta phi) (baz)))
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Error: The value of X, 1/3, is not ALPHA, OMEGA, ZETA, or PHI.
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To continue, type :CONTINUE followed by an option number:
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1: Return to Lisp Toplevel.
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Debug>
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</pre><P>
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>ccase <i>keyplace</i> {({({<i>key</i>}*) | <i>key</i>} {<i>form</i>}*)}*</tt><P> [This supersedes the description of <tt>ccase</tt>
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given in section <A HREF="node222.html#EXHAUSTIVECASEANALYSIS">24.3</A>.-GLS]
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<P>
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This control construct is similar to <tt>case</tt>, but no explicit <tt>otherwise</tt> or
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<tt>t</tt> clause is permitted.
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<P>
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The <i>keyplace</i> must be a generalized variable reference acceptable to <tt>setf</tt>.
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If no clause is satisfied, <tt>ccase</tt> signals an error (of type <tt>type-error</tt>)
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with a message constructed from the clauses. This error may be continued
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using the <tt>store-value</tt> restart. The argument to <tt>store-value</tt> is stored in
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<i>keyplace</i> and then <tt>ccase</tt> starts over, making the type tests again. Subforms
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of <i>keyplace</i> may be evaluated multiple times. If the <tt>store-value</tt> restart is
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invoked interactively, the user will be prompted for the value to be used.
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<P>
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The name of this function is mnemonic for ``continuable (exhaustive) case.''
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<P>
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<hr>
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<b>Implementation note:</b> The <tt>type-error</tt> signaled by <tt>ccase</tt> and <tt>ecase</tt> is free to
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choose any representation of the acceptable argument type that it wishes
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for placement in the expected-type slot. It will always work to use type
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<tt>(member . <i>keys</i>)</tt>, but in some cases it may be more efficient, for example,
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to use a type that represents an integer subrange or a type composed using the
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<tt>or</tt> type specifier.
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<hr>
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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=tex2html5908 HREF="node338.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5906 HREF="node334.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5900 HREF="node336.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5910 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5911 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5909 HREF="node338.html"> Handling Conditions</A>
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<B>Up:</B> <A NAME=tex2html5907 HREF="node334.html"> Program Interface to </A>
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<B> Previous:</B> <A NAME=tex2html5901 HREF="node336.html"> Assertions</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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</BODY>
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