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<title>Properties Template</title>
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<main role="main">
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<!--Weave of 'Properties Template' generated by Inweb-->
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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../extensions.html">Kits</a></li><li><a href="index.html">BasicInformKit</a></li><li><b>Properties Template</b></li></ul></div>
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<p class="purpose">Runtime access to property values.</p>
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<ul class="toc"><li><a href="S-prp.html#SP1">§1. Whether Provides</a></li><li><a href="S-prp.html#SP2">§2. Scan Property Metadata</a></li><li><a href="S-prp.html#SP3">§3. Read Property</a></li><li><a href="S-prp.html#SP4">§4. Write Property</a></li><li><a href="S-prp.html#SP5">§5. Printing Property Names</a></li></ul><hr class="tocbar">
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. Whether Provides. </b>This routine defines the phrase "if O provides P": there are three tests
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to pass, and if any of the three fail, we return <span class="extract"><span class="extract-syntax">false</span></span>. (The <span class="extract"><span class="extract-syntax">issue_rtp</span></span>
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flag, causing RTPs to be issued depending on which test fails, is never set
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when the routine is simply testing the condition.)
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</p>
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<p class="commentary">Firstly, P has to be a property known to I7. Secondly, there has to be
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permission either for this individual object to have it, or for its kind to
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have it, or its kind's kind, and so on. Thirdly, the object has to actually
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have the property in question at an I6 level — having permission to have
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it doesn't mean it actually does have.
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax">[ </span><span class="identifier-syntax">ProvidesProperty</span><span class="plain-syntax"> </span><span class="identifier-syntax">K</span><span class="plain-syntax"> </span><span class="identifier-syntax">obj</span><span class="plain-syntax"> </span><span class="identifier-syntax">q</span><span class="plain-syntax"> </span><span class="identifier-syntax">issue_rtp</span><span class="plain-syntax"> </span><span class="identifier-syntax">a</span><span class="plain-syntax"> </span><span class="identifier-syntax">l</span><span class="plain-syntax"> </span><span class="identifier-syntax">holder</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">K</span><span class="plain-syntax"> == </span><span class="identifier-syntax">OBJECT_TY</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">ScanPropertyMetadata</span><span class="plain-syntax">(</span><span class="identifier-syntax">obj</span><span class="plain-syntax">, </span><span class="identifier-syntax">q</span><span class="plain-syntax">, </span><span class="constant-syntax">4</span><span class="plain-syntax">)) </span><span class="reserved-syntax">jump</span><span class="plain-syntax"> </span><span class="identifier-syntax">PermissionFound</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">obj</span><span class="plain-syntax"> </span><span class="reserved-syntax">provides</span><span class="plain-syntax"> </span><span class="identifier-syntax">KD_Count</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">l</span><span class="plain-syntax"> = </span><span class="identifier-syntax">obj</span><span class="plain-syntax">.</span><span class="identifier-syntax">KD_Count</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">while</span><span class="plain-syntax"> (</span><span class="identifier-syntax">l</span><span class="plain-syntax"> > </span><span class="constant-syntax">0</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">a</span><span class="plain-syntax"> = </span><span class="identifier-syntax">l</span><span class="plain-syntax">*2;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">ScanPropertyMetadata</span><span class="plain-syntax">(</span><span class="identifier-syntax">KindHierarchy</span><span class="plain-syntax">--></span><span class="identifier-syntax">a</span><span class="plain-syntax">, </span><span class="identifier-syntax">q</span><span class="plain-syntax">, </span><span class="constant-syntax">4</span><span class="plain-syntax">)) </span><span class="reserved-syntax">jump</span><span class="plain-syntax"> </span><span class="identifier-syntax">PermissionFound</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">l</span><span class="plain-syntax"> = </span><span class="identifier-syntax">KindHierarchy</span><span class="plain-syntax">-->(</span><span class="identifier-syntax">a</span><span class="plain-syntax">+1);</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">ScanPropertyMetadata</span><span class="plain-syntax">(</span><span class="identifier-syntax">K0_kind</span><span class="plain-syntax">, </span><span class="identifier-syntax">q</span><span class="plain-syntax">, </span><span class="constant-syntax">4</span><span class="plain-syntax">)) </span><span class="reserved-syntax">jump</span><span class="plain-syntax"> </span><span class="identifier-syntax">PermissionFound</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">issue_rtp</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">IssueRTP</span><span class="plain-syntax">(</span><span class="string-syntax">"UnprovidedProperty"</span><span class="plain-syntax">,</span>
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<span class="plain-syntax"> </span><span class="string-syntax">"Attempt to use a property without permission."</span><span class="plain-syntax">, </span><span class="identifier-syntax">BasicInformKitRTPs</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">print</span><span class="plain-syntax"> </span><span class="string-syntax">"*** Since "</span><span class="plain-syntax">, (</span><span class="identifier-syntax">the</span><span class="plain-syntax">) </span><span class="identifier-syntax">obj</span><span class="plain-syntax">, </span><span class="string-syntax">" is not allowed the property ~"</span><span class="plain-syntax">,</span>
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<span class="plain-syntax"> (</span><span class="identifier-syntax">PrintPropertyName</span><span class="plain-syntax">) </span><span class="identifier-syntax">q</span><span class="plain-syntax">, </span><span class="string-syntax">"~, it can't be used.^"</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">rfalse</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> .</span><span class="identifier-syntax">PermissionFound</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">K</span><span class="plain-syntax"> >> </span><span class="identifier-syntax">obj</span><span class="plain-syntax"> </span><span class="reserved-syntax">provides</span><span class="plain-syntax"> </span><span class="identifier-syntax">q</span><span class="plain-syntax">) </span><span class="reserved-syntax">rtrue</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">issue_rtp</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">IssueRTP</span><span class="plain-syntax">(</span><span class="string-syntax">"UnsetProperty"</span><span class="plain-syntax">,</span>
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<span class="plain-syntax"> </span><span class="string-syntax">"Attempt to use a property with no value."</span><span class="plain-syntax">, </span><span class="identifier-syntax">BasicInformKitRTPs</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">print</span><span class="plain-syntax"> </span><span class="string-syntax">"*** Although "</span><span class="plain-syntax">, (</span><span class="identifier-syntax">the</span><span class="plain-syntax">) </span><span class="identifier-syntax">obj</span><span class="plain-syntax">, </span><span class="string-syntax">" is allowed to have the property ~"</span><span class="plain-syntax">,</span>
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<span class="plain-syntax"> (</span><span class="identifier-syntax">PrintPropertyName</span><span class="plain-syntax">) </span><span class="identifier-syntax">q</span><span class="plain-syntax">, </span><span class="string-syntax">"~, no value was ever given.^"</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">rfalse</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">];</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>§2. Scan Property Metadata. </b>The supplied metadata table is a series of zero-terminated lists of objects
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(or class objects, representing I7 kinds). Each such list corresponds to a single
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property; the position in the table is called the "offset" for the property.
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The following searches from a given offset.
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax">[ </span><span class="identifier-syntax">ScanPropertyMetadata</span><span class="plain-syntax"> </span><span class="identifier-syntax">obj</span><span class="plain-syntax"> </span><span class="identifier-syntax">property_metadata</span><span class="plain-syntax"> </span><span class="identifier-syntax">off</span><span class="plain-syntax"> </span><span class="identifier-syntax">i</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">for</span><span class="plain-syntax"> (</span><span class="identifier-syntax">i</span><span class="plain-syntax">=</span><span class="identifier-syntax">off</span><span class="plain-syntax">: </span><span class="identifier-syntax">property_metadata</span><span class="plain-syntax">--></span><span class="identifier-syntax">i</span><span class="plain-syntax"> >= </span><span class="constant-syntax">0</span><span class="plain-syntax">: </span><span class="identifier-syntax">i</span><span class="plain-syntax">++)</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">obj</span><span class="plain-syntax"> == </span><span class="identifier-syntax">property_metadata</span><span class="plain-syntax">--></span><span class="identifier-syntax">i</span><span class="plain-syntax">) </span><span class="reserved-syntax">rtrue</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">rfalse</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">];</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>§3. Read Property. </b>Some value properties belong to other values (those created in tables), and
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these are stored indirectly through a pseudo-object <span class="extract"><span class="extract-syntax">V</span></span> — an I6 object which
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is not part of the world model, and not a valid I7 "object" value, but which is
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used in order that properties belonging to values
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are still I6 property numbers. <span class="extract"><span class="extract-syntax">V.P</span></span> is the table column address for this
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property; <span class="extract"><span class="extract-syntax">obj</span></span> is then a value for the kind of value created by the table, so
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it is used as an index into the table column to get the address of the memory
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location storing the property value.
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax">[ </span><span class="identifier-syntax">GProperty</span><span class="plain-syntax"> </span><span class="identifier-syntax">K</span><span class="plain-syntax"> </span><span class="identifier-syntax">V</span><span class="plain-syntax"> </span><span class="identifier-syntax">pr</span><span class="plain-syntax"> </span><span class="identifier-syntax">err</span><span class="plain-syntax"> </span><span class="identifier-syntax">holder</span><span class="plain-syntax"> </span><span class="identifier-syntax">val</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">ProvidesProperty</span><span class="plain-syntax">(</span><span class="identifier-syntax">K</span><span class="plain-syntax">, </span><span class="identifier-syntax">V</span><span class="plain-syntax">, </span><span class="identifier-syntax">pr</span><span class="plain-syntax">, </span><span class="constant-syntax">1</span><span class="plain-syntax">-</span><span class="identifier-syntax">err</span><span class="plain-syntax">)) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> (</span><span class="identifier-syntax">K</span><span class="plain-syntax"> >> </span><span class="identifier-syntax">V</span><span class="plain-syntax"> . </span><span class="identifier-syntax">pr</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">0</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">];</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP4" class="paragraph-anchor"></a><b>§4. Write Property. </b>This routine's name must consist of the read-value-property routine's name
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with the prefix <span class="extract"><span class="extract-syntax">Write</span></span>, as that is how a reference to such a property is
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converted from an rvalue to an lvalue.
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax">[ </span><span class="identifier-syntax">WriteGProperty</span><span class="plain-syntax"> </span><span class="identifier-syntax">K</span><span class="plain-syntax"> </span><span class="identifier-syntax">V</span><span class="plain-syntax"> </span><span class="identifier-syntax">pr</span><span class="plain-syntax"> </span><span class="identifier-syntax">val</span><span class="plain-syntax"> </span><span class="identifier-syntax">holder</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">ProvidesProperty</span><span class="plain-syntax">(</span><span class="identifier-syntax">K</span><span class="plain-syntax">, </span><span class="identifier-syntax">V</span><span class="plain-syntax">, </span><span class="identifier-syntax">pr</span><span class="plain-syntax">, </span><span class="reserved-syntax">true</span><span class="plain-syntax">)) {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">K</span><span class="plain-syntax"> >> </span><span class="identifier-syntax">V</span><span class="plain-syntax"> . </span><span class="identifier-syntax">pr</span><span class="plain-syntax"> = </span><span class="identifier-syntax">val</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax">];</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP5" class="paragraph-anchor"></a><b>§5. Printing Property Names. </b>Inform doesn't print property names prettily; it more or less prints them
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only as decimal numbers.
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax">[ </span><span class="identifier-syntax">PROPERTY_TY_Say</span><span class="plain-syntax"> </span><span class="identifier-syntax">v</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">print</span><span class="plain-syntax"> </span><span class="string-syntax">"property "</span><span class="plain-syntax">, </span><span class="identifier-syntax">v</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">];</span>
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<span class="plain-syntax">[ </span><span class="identifier-syntax">PrintPropertyName</span><span class="plain-syntax"> </span><span class="identifier-syntax">p</span><span class="plain-syntax"> </span><span class="identifier-syntax">textual</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">p</span><span class="plain-syntax"><0) </span><span class="identifier-syntax">p</span><span class="plain-syntax"> = ~</span><span class="identifier-syntax">p</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">textual</span><span class="plain-syntax"> = </span><span class="identifier-syntax">p</span><span class="plain-syntax">-->3;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">print</span><span class="plain-syntax"> (</span><span class="reserved-syntax">string</span><span class="plain-syntax">) </span><span class="identifier-syntax">textual</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">];</span>
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