398 lines
8.3 KiB
HTML
398 lines
8.3 KiB
HTML
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<!DOCTYPE HTML>
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<HTML LANG="en-us"
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><HEAD
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><TITLE
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>throw | Common Lisp Nova Spec</TITLE
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><META CHARSET="US-ASCII"
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><LINK REL="canonical" HREF="f_throw.html"
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><LINK REL="next" HREF="f_unwind-protect.html" TYPE="text/html" TITLE="unwind-protect"
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><LINK REL="prev" HREF="f_tagbody.html" TYPE="text/html" TITLE="tagbody"
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><LINK REL="up" HREF="5_3_Data_and_Control_Flow_Dictionary.html" TYPE="text/html" TITLE="5.3 Data and Control Flow Dictionary"
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><LINK REL="start" HREF="index.html" TYPE="text/html" TITLE="Common Lisp Nova Spec"
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><META NAME="VIEWPORT" CONTENT="width=device-width, initial-scale=1.0"
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></SCRIPT
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><SCRIPT SRC="apropos.js%3F3909942064"
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></SCRIPT
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></HEAD
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><BODY
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><DIV
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><DIV CLASS="topnav"
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><DIV CLASS="breadcrumb"
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><SPAN CLASS="breadcrumb-item"
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><A HREF="index.html"
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>Common Lisp Nova Spec</A
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></SPAN
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="5_Data_and_Control_Flow.html"
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>5. Data and Control Flow</A
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></SPAN
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="5_3_Data_and_Control_Flow_Dictionary.html"
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>5.3 Data and Control Flow Dictionary</A
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></SPAN
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="f_throw.html"
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>throw</A
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></SPAN
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></DIV
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><DIV CLASS="apropos"
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><DIV CLASS="apropos-io"
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><A HREF="f_tagbody.html" CLASS="prev"
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>←</A
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><SPAN ID="apropos-label"
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>Apropos </SPAN
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><INPUT ID="apropos" AUTOFOCUS="AUTOFOCUS" PLACEHOLDER="Type here to search" ONINPUT="aproposInput(this);" ONKEYUP="aproposKeyup(event);" ONCHANGE="aproposChange(this);" ONFOCUS="aproposFocus(this);" ONFOCUSOUT="aproposFocusout(this);"
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><A HREF="f_unwind-protect.html" CLASS="next"
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>→</A
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></DIV
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><DIV ID="apropos-res"
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></DIV
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></DIV
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></DIV
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><DIV CLASS="matter"
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><DIV CLASS="com"
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><DIV CLASS="begincom"
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><HR
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><TABLE WIDTH="100%" CELLSPACING="0" CELLPADDING="0"
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><TR
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><TD ALIGN="LEFT" VALIGN="BASELINE" WIDTH="100%" CLASS="name"
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><SPAN CLASS="idx" DATA-KIND="idxref" DATA-TERM="throw"
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></SPAN
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><B
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>throw</B
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></TD
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><TD ALIGN="RIGHT" VALIGN="BASELINE" WIDTH="0" NOWRAP="NOWRAP" CLASS="ftype"
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><I
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>Special</I
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> <I
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>Operator</I
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></TD
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></TR
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></TABLE
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><HR
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></DIV
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><UL CLASS="subtoc"
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></UL
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><DL
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><DT
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><B
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>Syntax</B
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></DT
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><DD
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><P CLASS="j"
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><B
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>throw</B
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> <SPAN CLASS="cmssi"
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>tag</SPAN
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> <SPAN CLASS="cmssi"
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>result-form</SPAN
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> <SPAN CLASS="arrow"
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>→</SPAN
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>|</P
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></DD
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><DT
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><B
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>Arguments and Values</B
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></DT
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><DD
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><P CLASS="j"
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><VAR CLASS="param"
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>tag</VAR
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> — a <A HREF="26_1_Glossary.html#catch_tag"
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><EM CLASS="term"
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>catch tag</EM
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></A
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>; evaluated. </P
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><P CLASS="j"
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><VAR CLASS="param"
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>result-form</VAR
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> — a <A HREF="26_1_Glossary.html#form"
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><EM CLASS="term"
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>form</EM
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></A
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>; evaluated as described below.</P
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></DD
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><DT
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><B
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>Description</B
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></DT
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><DD
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><P CLASS="j"
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><A HREF="f_throw.html" CLASS="specref"
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><B
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>throw</B
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></A
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> causes a non-local control transfer to a <A HREF="f_catch.html" CLASS="specref"
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><B
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>catch</B
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></A
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> whose tag is <A HREF="f_eq.html" CLASS="funref"
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><B
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>eq</B
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></A
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> to <VAR CLASS="param"
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>tag</VAR
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>. </P
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><P CLASS="j"
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><VAR CLASS="param"
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>Tag</VAR
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> is evaluated first to produce an <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>object</EM
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></A
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> called the throw tag; then <VAR CLASS="param"
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>result-form</VAR
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> is evaluated, and its results are saved. If the <VAR CLASS="param"
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>result-form</VAR
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> produces multiple values, then all the values are saved. The most recent outstanding <A HREF="f_catch.html" CLASS="specref"
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><B
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>catch</B
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></A
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> whose <VAR CLASS="param"
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>tag</VAR
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> is <A HREF="f_eq.html" CLASS="funref"
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><B
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>eq</B
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></A
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> to the throw tag is exited; the saved results are returned as the value or values of <A HREF="f_catch.html" CLASS="specref"
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><B
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>catch</B
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></A
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>. </P
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><P CLASS="j"
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>The transfer of control initiated by <A HREF="f_throw.html" CLASS="specref"
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><B
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>throw</B
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></A
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> is performed as described in <A HREF="5_2_Transfer_of_Control_to_an_Exit_Point.html#sec_5_2" CLASS="secref"
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><SPAN CLASS="cmr"
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>Section</SPAN
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> <SPAN CLASS="cmr"
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>5.2</SPAN
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> <SPAN CLASS="cmr"
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>(Transfer</SPAN
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> <SPAN CLASS="cmr"
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>of</SPAN
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> <SPAN CLASS="cmr"
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>Control</SPAN
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> <SPAN CLASS="cmr"
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>to</SPAN
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> <SPAN CLASS="cmr"
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>an</SPAN
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> <SPAN CLASS="cmr"
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>Exit</SPAN
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> <SPAN CLASS="cmr"
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>Point)</SPAN
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></A
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>.</P
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></DD
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><DT
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><B
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>Examples</B
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></DT
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><DD
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><PRE CLASS="screen"
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>(catch 'result
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(setq i 0 j 0)
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(loop (incf j 3) (incf i)
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(if (= i 3) (throw 'result (values i j))))) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 3, 9</PRE
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><PRE CLASS="screen"
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>(catch nil
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(unwind-protect (throw nil 1)
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(throw nil 2))) <SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 2</PRE
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><P CLASS="j"
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>The consequences of the following are undefined because the <A HREF="f_catch.html" CLASS="specref"
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><B
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>catch</B
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></A
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> of <CODE CLASS="f"
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>b</CODE
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> is passed over by the first <A HREF="f_throw.html" CLASS="specref"
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><B
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>throw</B
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></A
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>, hence portable programs must assume that its <A HREF="26_1_Glossary.html#dynamic_extent"
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><EM CLASS="term"
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>dynamic extent</EM
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></A
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> is terminated. The <A HREF="26_1_Glossary.html#binding"
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><EM CLASS="term"
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>binding</EM
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></A
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> of the <A HREF="26_1_Glossary.html#catch_tag"
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><EM CLASS="term"
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>catch tag</EM
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></A
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> is not yet <A HREF="26_1_Glossary.html#disestablish"
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><EM CLASS="term"
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>disestablished</EM
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></A
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> and therefore it is the target of the second <A HREF="f_throw.html" CLASS="specref"
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><B
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>throw</B
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></A
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>.</P
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><PRE CLASS="screen"
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>(catch 'a
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(catch 'b
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(unwind-protect (throw 'a 1)
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(throw 'b 2))))</PRE
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><P CLASS="j"
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>The following prints “<CODE CLASS="f"
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>The inner catch returns :SECOND-THROW</CODE
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>” and then returns <CODE CLASS="f"
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>:outer-catch</CODE
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>.</P
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><PRE CLASS="screen"
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> (catch 'foo
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(format t "The inner catch returns ~s.~%"
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(catch 'foo
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(unwind-protect (throw 'foo :first-throw)
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(throw 'foo :second-throw))))
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:outer-catch)
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> The inner catch returns :SECOND-THROW
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<SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> :OUTER-CATCH</PRE
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></DD
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><DT
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><B
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>Exceptional Situations</B
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></DT
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><DD
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><P CLASS="j"
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>If there is no outstanding <A HREF="26_1_Glossary.html#catch_tag"
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><EM CLASS="term"
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>catch tag</EM
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></A
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> that matches the throw tag, no unwinding of the stack is performed, and an error of <A HREF="26_1_Glossary.html#type"
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><EM CLASS="term"
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>type</EM
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></A
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> <A HREF="t_control-error.html" CLASS="typeref"
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><B
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>control-error</B
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></A
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> is signaled. When the error is signaled, the <A HREF="26_1_Glossary.html#dynamic_environment"
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><EM CLASS="term"
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>dynamic environment</EM
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></A
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> is that which was in force at the point of the <A HREF="f_throw.html" CLASS="specref"
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><B
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>throw</B
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></A
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>.</P
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></DD
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><DT
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><B
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>See Also</B
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></DT
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><DD
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><P CLASS="j"
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><A HREF="f_block.html" CLASS="specref"
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><B
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>block</B
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></A
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>, <A HREF="f_catch.html" CLASS="specref"
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><B
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>catch</B
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></A
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>, <A HREF="f_return-from.html" CLASS="specref"
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><B
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>return-from</B
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></A
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>, <A HREF="f_unwind-protect.html" CLASS="specref"
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><B
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>unwind-protect</B
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></A
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>, <A HREF="3_1_Evaluation.html#sec_3_1" CLASS="secref"
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><SPAN CLASS="cmr"
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>Section</SPAN
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> <SPAN CLASS="cmr"
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>3.1</SPAN
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> <SPAN CLASS="cmr"
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>(Evaluation)</SPAN
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></A
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></P
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></DD
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><DT
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><B
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>Notes</B
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></DT
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><DD
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><P CLASS="j"
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><A HREF="f_catch.html" CLASS="specref"
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><B
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>catch</B
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></A
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> and <A HREF="f_throw.html" CLASS="specref"
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><B
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>throw</B
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></A
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> are normally used when the <A HREF="26_1_Glossary.html#exit_point"
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><EM CLASS="term"
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>exit point</EM
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></A
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> must have <A HREF="26_1_Glossary.html#dynamic_scope"
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><EM CLASS="term"
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>dynamic scope</EM
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></A
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> (<I CLASS="i"
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><I
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>e.g.</I
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></I
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>, the <A HREF="f_throw.html" CLASS="specref"
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><B
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>throw</B
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></A
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> is not lexically enclosed by the <A HREF="f_catch.html" CLASS="specref"
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><B
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>catch</B
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></A
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>), while <A HREF="f_block.html" CLASS="specref"
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><B
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>block</B
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></A
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> and <A HREF="f_return.html" CLASS="macref"
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><B
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>return</B
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></A
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> are used when <A HREF="26_1_Glossary.html#lexical_scope"
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><EM CLASS="term"
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>lexical scope</EM
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></A
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> is sufficient.</P
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></DD
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></DL
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></DIV
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></DIV
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><DIV CLASS="footer"
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><DIV CLASS="btmnav"
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><A HREF="f_tagbody.html" CLASS="prev"
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>←</A
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><A HREF="f_unwind-protect.html" CLASS="next"
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>→</A
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></DIV
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><DIV CLASS="trail"
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>Conversion to HTML copyright 2023 by Gilbert Baumann</DIV
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></DIV
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></DIV
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><SCRIPT
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>domReady();</SCRIPT
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></BODY
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></HTML
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>
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