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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>3.4 Expression Wrapper: #%expression</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">►</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">The Racket Reference</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewselflink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="runnin
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<span class="RktSym"><a href="__expression.html#%28form._%28%28quote._~23~25kernel%29._~23~25expression%29%29" class="RktStxLink" data-pltdoc="x">#%expression</a></span> forces the parsing of a form as an
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expression.</div></p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="__expression.html#%28form._%28%28quote._~23~25kernel%29._~23~25expression%29%29" class="RktStxLink" data-pltdoc="x">#%expression</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">3</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="__expression.html#%28form._%28%28quote._~23~25kernel%29._~23~25expression%29%29" class="RktStxLink" data-pltdoc="x">#%expression</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktVal">10</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">eval:8:0: define: not allowed in an expression context</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (define x 10)</span></p></td></tr></table></blockquote></div></p><p><div class="SIntrapara">The <span class="RktSym"><a href="__expression.html#%28form._%28%28quote._~23~25kernel%29._~23~25expression%29%29" class="RktStxLink" data-pltdoc="x">#%expression</a></span> form is helpful in recursive definition contexts
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where expanding a subsequent definition can provide compile-time information
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for the current expression. For example, consider a <span class="RktSym">define-sym-case</span>
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macro that simply records some symbols at compile-time in a given identifier.
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</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">define-sym-case</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=Library_Syntax_Classes_and_Literal_Sets.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._id%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">id</a></span><span class="hspace"> </span><span class="RktSym">sym</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._andmap%29%29" class="RktValLink" data-pltdoc="x">andmap</a></span><span class="hspace"> </span><span class="RktSym"><a href="stxops.html#%28def._%28%28lib._racket%2Fprivate%2Fstx..rkt%29._identifier~3f%29%29" class="RktValLink" data-pltdoc="x">identifier?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3elist%29%29" class="RktValLink" data-pltdoc="x">syntax->list</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">sym</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=Library_Syntax_Classes_and_Literal_Sets.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._id%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">id</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">sym</span><span class="hspace"> </span><span class="RktVal">...</span><span class="RktVal">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)
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used in the expression match those given in the earlier definition:
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</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">sym-case</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=Library_Syntax_Classes_and_Literal_Sets.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._id%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">id</a></span><span class="hspace"> </span><span class="RktSym">val-expr</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">sym</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">expected-ids</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stxtrans.html#%28def._%28%28quote._~23~25kernel%29._syntax-local-value%29%29" class="RktValLink" data-pltdoc="x">syntax-local-value</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=Library_Syntax_Classes_and_Literal_Sets.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._id%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">id</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._~ce~bb%29%29" class="RktStxLink" data-pltdoc="x">λ</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> &n
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the <span class="RktSym">define-sym-case</span> macro does not have
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a chance to bind <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=Library_Syntax_Classes_and_Literal_Sets.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._id%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">id</a></span> and the <span class="RktSym">sym-case</span>
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macro signals an error:
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</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">sym-case</span><span class="hspace"> </span><span class="RktSym">land-creatures</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">bear</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">bear</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">fox</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">define-sym-case</span><span class="hspace"> </span><span class="RktSym">land-creatures</span><span class="hspace"> </span><span class="RktSym">bear</span><span class="hspace"> </span><span class="RktSym">fox</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktErr">eval:11:0: sym-case: expected an identifier bound via</span></p></td></tr><tr><td><p><span class="RktErr">define-sym-case</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: land-creatures</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (sym-case land-creatures (quote bear) ((bear) 1)</span></p></td></tr><tr><td><p><span class="RktErr">((fox) 2))</span></p></td></tr></table></blockquote></div><div class="SIntrapara">But if the <span class="RktSym">sym-case</span> is wrapped in an <span class="RktSym"><a href="__expression.html#%28form._%28%28quote._~23~25kernel%29._~23~25expression%29%29" class="RktStxLink" data-pltdoc="x">#%expression</a></span>,
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then the expander does not need to expand it to know it is
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an expression and it moves on to the <span class="RktSym">define-sym-case</span>
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expression.
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</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="__expression.html#%28form._%28%28quote._~23~25kernel%29._~23~25expression%29%29" class="RktStxLink" data-pltdoc="x">#%expression</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">sym-case</span><span class="hspace"> </span><span class="RktSym">sea-creatures</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">whale</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">whale</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">squid</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">define-sym-case</span><span class="hspace"> </span><span class="RktSym">sea-creatures</span><span class="hspace"> </span><span class="RktSym">whale</span><span class="hspace"> </span><span class="RktSym">squid</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">more...</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'more...</span></p></td></tr></table></blockquote></div><div class="SIntrapara">Of course, a macro like <span class="RktSym">sym-case</span> should not require its
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clients to add <span class="RktSym"><a href="__expression.html#%28form._%28%28quote._~23~25kernel%29._~23~25expression%29%29" class="RktStxLink" data-pltdoc="x">#%expression</a></span>; instead it should check
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the basic shape of its arguments and then expand to <span class="RktSym"><a href="__expression.html#%28form._%28%28quote._~23~25kernel%29._~23~25expression%29%29" class="RktStxLink" data-pltdoc="x">#%expression</a></span>
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wrapped around a helper macro that calls <span class="RktSym"><a href="stxtrans.html#%28def._%28%28quote._~23~25kernel%29._syntax-local-value%29%29" class="RktValLink" data-pltdoc="x">syntax-local-value</a></span>
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and finishes the expansion.</div></p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="quote.html" title="backward to "3.3 Literals: quote and #%datum"" data-pltdoc="x">← prev</a> <a href="syntax.html" title="up to "3 Syntactic Forms"" data-pltdoc="x">up</a> <a href="__top.html" title="forward to "3.5 Variable References and #%top"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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