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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>3.11 Constructing Graphs: shared</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="running.ht
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and then evaluates the <span class="RktVar">body</span>s, returning the result of
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the last expression.</div></p><p>The <span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span> form is similar to <span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._letrec%29%29" class="RktStxLink" data-pltdoc="x">letrec</a></span>, except that
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special forms of <span class="RktVar">expr</span> are recognized (after partial macro
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expansion) to construct graph-structured data, where the corresponding
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<span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._letrec%29%29" class="RktStxLink" data-pltdoc="x">letrec</a></span> would instead produce a use-before-initialization error.</p><p>Each <span class="RktVar">expr</span> (after partial expansion) is matched against the
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following <span class="RktVar">shared-expr</span> grammar, where earlier variants in a
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production take precedence over later variants:</p><p><table cellspacing="0" cellpadding="0"><tr><td align="right" valign="baseline"><span class="hspace"> </span><span class="RktVar">shared-expr</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">shell-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">plain-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td></tr><tr><td align="right" valign="baseline"><span class="hspace"> </span><span class="RktVar">shell-expr</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktVar">in-immutable-expr</span><span class="hspace"> </span><span class="RktVar">in-immutable-expr</span><span class="RktPn">)</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._list%29%29" class="RktValLink" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVar">in-immutable-expr</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._list%2A%29%29" class="RktValLink" data-pltdoc="x">list*</a></span><span class="hspace"> </span><span class="RktVar">in-immutable-expr</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._append%29%29" class="RktValLink" data-pltdoc="x">append</a></span><span class="hspace"> </span><span class="RktVar">early-expr</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="hspace"> </span><span class="RktVar">in-immutable-expr</span><span class="RktPn">)</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" v
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first kind is any binding whose name has <span class="RktSym">make-</span> in the
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middle, and where <span class="RktVar">prefix:</span><span class="RktVar">id</span> has a <a href="syntax-model.html#%28tech._transformer%29" class="techoutside" data-pltdoc="x"><span class="techinside">transformer</span></a> binding to
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structure information with a full set of mutator bindings; see
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<a href="structinfo.html" data-pltdoc="x">Structure Type Transformer Binding</a>. The second kind is an identifier that itself has a
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<a href="syntax-model.html#%28tech._transformer%29" class="techoutside" data-pltdoc="x"><span class="techinside">transformer</span></a> binding to structure information. The third kind is an
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identifier that has a <span class="RktVal">'</span><span class="RktVal">constructor-for</span> <a href="stxprops.html#%28tech._syntax._property%29" class="techoutside" data-pltdoc="x"><span class="techinside">syntax property</span></a>
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whose value is an identifier with a <a href="syntax-model.html#%28tech._transformer%29" class="techoutside" data-pltdoc="x"><span class="techinside">transformer</span></a> binding to structure
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information. A <span class="RktVar">shell-id</span>, meanwhile, must be one of the
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<span class="RktVar">id</span>s bound by the <span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span> form to a
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<span class="RktVar">shell-expr</span>.</p><p>When the <span class="RktVar">expr</span>s of the <span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span> form are parsed as
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<span class="RktVar">shared-expr</span> (taking into account the order of the variants
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for parsing precedence), the sub-expressions that were parsed via
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<span class="RktVar">early-expr</span> will be evaluated first when the <span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span>
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form is evaluated. Among such expressions, they are evaluated in the
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order as they appear within the <span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span> form. However, any
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reference to an <span class="RktVar">id</span> bound by <span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span> produces
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a use-before-initialization errror, even if the binding for the <span class="RktVar">id</span> appears
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before the corresponding <span class="RktVar">early-expr</span> within the
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<span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span> form.</p><p>The <span class="RktVar">shell-id</span>s and <span class="RktVar">shell-expr</span>s (not counting
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<span class="RktVar">patchable-expr</span> and <span class="RktVar">early-expr</span> sub-expressions) are
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effectively evaluated next:</p><ul><li><p>A <span class="RktVar">shell-id</span> reference produces the same value as the
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corresponding <span class="RktVar">id</span> will produce within the
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<span class="RktVar">body</span>s, assuming that <span class="RktVar">id</span> is never mutated
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with <span class="RktSym"><a href="set_.html#%28form._%28%28quote._~23~25kernel%29._set%21%29%29" class="RktStxLink" data-pltdoc="x">set!</a></span>. This special handling of a
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<span class="RktVar">shell-id</span> reference is one way in which
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<span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span> supports the creation of cyclic data, including
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immutable cyclic data.</p></li><li><p>A <span class="RktVar">shell-expr</span> of the form <span class="RktPn">(</span><span class="RktSym"><a href="mpairs.html#%28def._%28%28quote._~23~25kernel%29._mcons%29%29" class="RktValLink" data-pltdoc="x">mcons</a></span><span class="stt"> </span><span class="RktVar">patchable-expr</span><span class="stt"> </span><span class="RktVar">patchable-expr</span><span class="RktPn">)</span>, <span class="RktPn">(</span><span class="RktSym"><a href="vectors.html#%28def._%28%28quote._~23~25kernel%29._vector%29%29" class="RktValLink" data-pltdoc="x">vector</a></span><span class="stt"> </span><span class="RktVar">patchable-expr</span><span class="stt"> </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="RktSym"><a href="boxes.html#%28def._%28%28quote._~23~25kernel%29._box%29%29" class="RktValLink" data-pltdoc="x">box</a></span><span class="stt"> </span><span class="RktVar">patchable-expr</span><span class="RktPn">)</span>, or <span class="RktPn">(</span><span class="RktVar">prefix:</span><span class="RktSym">make-</span><span class="RktVar">id</span><span class="stt"> </span><span class="RktVar">patchable-expr</span><span class="stt"> </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> produces a mutable value whose content
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positions are initialized to <span class="RktSym"><a href="undefined.html#%28def._%28%28lib._racket%2Fundefined..rkt%29._undefined%29%29" class="RktValLink" data-pltdoc="x">undefined</a></span>. Each content
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position is <a name="(tech._patched)"></a><span style="font-style: italic">patched</span> (i.e., updated) after the
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corresponding <span class="RktVar">patchable-expr</span> expression is later
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evaluated.</p></li></ul><p>Next, the <span class="RktVar">plain-expr</span>s are evaluated as for <span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._letrec%29%29" class="RktStxLink" data-pltdoc="x">letrec</a></span>,
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where a reference to an <span class="RktVar">id</span> raises <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract~3avariable%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract:variable</a></span> if it
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is evaluated before the right-hand side of the <span class="RktVar">id</span> binding.</p><p>Finally, the <span class="RktVar">patchable-expr</span>s are evaluated and their values
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replace <span class="RktSym"><a href="undefined.html#%28def._%28%28lib._racket%2Fundefined..rkt%29._undefined%29%29" class="RktValLink" data-pltdoc="x">undefined</a></span>s in the results of
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<span class="RktVar">shell-expr</span>s. At this point, all <span class="RktVar">id</span>s are bound, so
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<span class="RktVar">patchable-expr</span>s can create data cycles (but only with cycles
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that can be created via mutation).</p><p><div class="SIntrapara">Examples:</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="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktSym">a</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="RktSym">a</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#0='(1 . #0#)</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktSym">b</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">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktSym">a</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="RktSym">a</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#0='(1 2 . #0#)</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktSym">b</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">b</span><span class="hspace"> </span><span class="RktVal">7</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span>
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