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<title>What This Module Does</title>
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<h1><a href="../index.html">
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<img src="../docs-assets/Inform.png" height=72">
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</a></h1>
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<ul><li><a href="../index.html">home</a></li>
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</ul><h2>Compiler</h2><ul>
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<li><a href="../structure.html">structure</a></li>
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<li><a href="../inbuildn.html">inbuild</a></li>
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<li><a href="../inform7n.html">inform7</a></li>
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<li><a href="../intern.html">inter</a></li>
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<li><a href="../services.html">services</a></li>
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<li><a href="../secrets.html">secrets</a></li>
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</ul><h2>Other Tools</h2><ul>
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<li><a href="../inblorbn.html">inblorb</a></li>
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<li><a href="../inform6.html">inform6</a></li>
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<li><a href="../inpolicyn.html">inpolicy</a></li>
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<li><a href="../inrtpsn.html">inrtps</a></li>
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</ul><h2>Resources</h2><ul>
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<li><a href="../extensions.html">extensions</a></li>
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<li><a href="../kits.html">kits</a></li>
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</ul><h2>Repository</h2><ul>
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<li><a href="https://github.com/ganelson/inform"><img src="../docs-assets/github.png" height=18> github</a></li>
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</ul><h2>Related Projects</h2><ul>
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<li><a href="../../../inweb/index.html">inweb</a></li>
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<li><a href="../../../intest/index.html">intest</a></li>
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</ul>
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</nav>
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<main role="main">
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<!--Weave of 'What This Module Does' generated by Inweb-->
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<div class="breadcrumbs">
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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../services.html">Services</a></li><li><a href="index.html">syntax</a></li><li><a href="index.html#P">Preliminaries</a></li><li><b>What This Module Does</b></li></ul></div>
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<p class="purpose">An overview of the syntax module's role and abilities.</p>
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<ul class="toc"><li><a href="P-wtmd.html#SP1">§1. Prerequisites</a></li><li><a href="P-wtmd.html#SP2">§2. Syntax trees</a></li><li><a href="P-wtmd.html#SP6">§6. Nodes</a></li><li><a href="P-wtmd.html#SP7">§7. Fussy, defensive, pedantry</a></li></ul><hr class="tocbar">
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. Prerequisites. </b>The syntax module is a part of the Inform compiler toolset. It is
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presented as a literate program or "web". Before diving in:
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</p>
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<ul class="items"><li>(a) It helps to have some experience of reading webs: see <a href="../../../inweb/index.html" class="internal">inweb</a> for more.
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</li><li>(b) The module is written in C, in fact ANSI C99, but this is disguised by the
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fact that it uses some extension syntaxes provided by the <a href="../../../inweb/index.html" class="internal">inweb</a> literate
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programming tool, making it a dialect of C called InC. See <a href="../../../inweb/index.html" class="internal">inweb</a> for
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full details, but essentially: it's C without predeclarations or header files,
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and where functions have names like <span class="extract"><span class="extract-syntax">Tags::add_by_name</span></span> rather than <span class="extract"><span class="extract-syntax">add_by_name</span></span>.
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</li><li>(c) This module uses other modules drawn from the compiler (see <a href="../structure.html" class="internal">structure</a>), and also
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uses a module of utility functions called <a href="../../../inweb/foundation-module/index.html" class="internal">foundation</a>.
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For more, see <a href="../../../inweb/foundation-module/P-abgtf.html" class="internal">A Brief Guide to Foundation (in foundation)</a>.
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</li></ul>
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<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>§2. Syntax trees. </b>Most algorithms for parsing natural language involve the construction of
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trees, in which the original words appear as leaves at the top of the tree,
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while the grammatical functions they serve appear as the branches and trunk:
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thus the word "orange", as an adjective, might be growing from a branch
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which represents a noun clause ("the orange envelope"), growing in turn from
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a trunk which in turn might represent a assertion sentence:
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</p>
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<blockquote>
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<p>The card is in the orange envelope.</p>
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</blockquote>
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<p class="commentary">The Inform tools represent syntax trees by <a href="2-st.html#SP2" class="internal">parse_node_tree</a> structures
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(see <a href="2-st.html#SP2" class="internal">SyntaxTree::new</a>), but there are very few of these: the entire
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source text compiled by <a href="../inform7/index.html" class="internal">inform7</a> is just one syntax tree. When <a href="../supervisor-module/index.html" class="internal">supervisor</a>
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manages extensions, it may generate one <a href="2-st.html#SP2" class="internal">parse_node_tree</a> object for each
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extension whose text it reads. Still — there are few trees.
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</p>
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<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>§3. </b>The trunk of the tree can be grown in any sequence: call <a href="2-st.html#SP3" class="internal">SyntaxTree::push_bud</a>
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to begin "budding" from a particular branch, and <a href="2-st.html#SP3" class="internal">SyntaxTree::pop_bud</a> to go back
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to where you were. These are also used automatically to ensure that sentences
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arriving at <a href="2-st.html#SP4" class="internal">SyntaxTree::graft_sentence</a> are grafted under the headings to
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which they belong. Thus, the sentences
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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">Chapter</span><span class="plain-syntax"> </span><span class="constant-syntax">20</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">Section</span><span class="plain-syntax"> </span><span class="constant-syntax">1</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">The</span><span class="plain-syntax"> </span><span class="identifier-syntax">cat</span><span class="plain-syntax"> </span><span class="identifier-syntax">is</span><span class="plain-syntax"> </span><span class="identifier-syntax">in</span><span class="plain-syntax"> </span><span class="identifier-syntax">the</span><span class="plain-syntax"> </span><span class="identifier-syntax">cardboard</span><span class="plain-syntax"> </span><span class="identifier-syntax">box</span><span class="plain-syntax">.</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">Section</span><span class="plain-syntax"> </span><span class="constant-syntax">2</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">The</span><span class="plain-syntax"> </span><span class="identifier-syntax">ball</span><span class="plain-syntax"> </span><span class="identifier-syntax">of</span><span class="plain-syntax"> </span><span class="identifier-syntax">yarn</span><span class="plain-syntax"> </span><span class="identifier-syntax">is</span><span class="plain-syntax"> </span><span class="identifier-syntax">here</span><span class="plain-syntax">.</span>
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</pre>
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<p class="commentary">would actually be grafted like so:
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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"> RESULT BUD STACK BEFORE THIS</span>
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<span class="plain-syntax"> Chapter 20 (empty)</span>
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<span class="plain-syntax"> Section 1 Chapter 20</span>
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<span class="plain-syntax"> The cat is in the cardboard box. Chapter 20 > Section 1</span>
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<span class="plain-syntax"> Section 2 Chapter 20 > Section 1</span>
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<span class="plain-syntax"> The ball of yarn is here. Chapter 20 > Section 2</span>
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</pre>
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<p class="commentary">But it is also possible to graft smaller (not-whole-sentence) cuttings onto
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each other using <a href="2-st.html#SP6" class="internal">SyntaxTree::graft</a>, which doesn't involve the bud stack
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at all.
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</p>
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<p class="commentary firstcommentary"><a id="SP4" class="paragraph-anchor"></a><b>§4. </b>Meaning is an ambiguous thing, and so the tree needs to be capable of
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representing multiple interpretations of the same wording. So nodes have not
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only <span class="extract"><span class="extract-syntax">next</span></span> and <span class="extract"><span class="extract-syntax">down</span></span> links to other nodes, but also <span class="extract"><span class="extract-syntax">next_alternative</span></span> links,
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which — if used — fork the syntax tree into different possible readings.
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</p>
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<p class="commentary">These are not added to the tree by grafting: that's only done for definite
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meanings. Instead, multiple ambiguous readings mostly lie beneath <span class="extract"><span class="extract-syntax">AMBIGUITY_NT</span></span>
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nodes — see <a href="2-st.html#SP21" class="internal">SyntaxTree::add_reading</a>. For example, we might 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"> sun is orange</span>
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<span class="plain-syntax"> sun</span>
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<span class="plain-syntax"> AMBIGUITY</span>
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<span class="plain-syntax"> orange (read as being a fruit)</span>
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<span class="plain-syntax"> orange (read as being a colour)</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP5" class="paragraph-anchor"></a><b>§5. </b>An extensive suite of functions is provided to make it easy to traverse
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a syntax tree, calling a visitor function on each node: see <a href="2-st.html#SP10" class="internal">SyntaxTree::traverse</a>.
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</p>
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<p class="commentary firstcommentary"><a id="SP6" class="paragraph-anchor"></a><b>§6. Nodes. </b>Syntax trees are made up of <a href="2-pn.html#SP1" class="internal">parse_node</a> structures. While these are in
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principle individual nodes, they effectively represent subtrees, because they
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carry with them links to the nodes below. A <a href="2-pn.html#SP1" class="internal">parse_node</a> object can
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therefore equally represent "orange", "the orange envelope", or "now the card
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is in the orange envelope".
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</p>
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<p class="commentary">Each node carries three essential pieces of information with it:
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</p>
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<ul class="items"><li>(1) The text giving rise to it (say, "Section Five - Fruit").
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</li><li>(2) A node type ID, which in broad terms says what kind of reference is being
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made (say, <span class="extract"><span class="extract-syntax">HEADING_NT</span></span>). The possible node types are stored in the C type
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<span class="extract"><span class="extract-syntax">node_type_t</span></span>, which corresponds to some metadata in a <a href="2-nt.html#SP3" class="internal">node_type_metadata</a>
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object: see <a href="2-pn.html#SP5" class="internal">Node::get_type</a> and <a href="2-nt.html#SP7" class="internal">NodeType::get_metadata</a>.
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</li><li>(3) A list of optional annotations, which are either integer or object-valued,
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and which give specifics about the meaning (say, the level number in the
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hierarchy of headings). See <a href="2-na.html" class="internal">Node Annotations</a>.
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</li></ul>
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<p class="commentary firstcommentary"><a id="SP7" class="paragraph-anchor"></a><b>§7. Fussy, defensive, pedantry. </b>Safe to say that Inform includes bugs: the more defensive coding we can do,
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the better. That means not only extensive logging (see <a href="2-pn.html#SP16" class="internal">Node::log_tree</a>)
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but also strict verification tests on every tree made (see <a href="2-tv.html" class="internal">Tree Verification</a>).
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</p>
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<ul class="items"><li>(a) The only nodes allowed to exist are those for node types declared
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by <a href="2-nt.html#SP9" class="internal">NodeType::new</a>: more generally, see <a href="2-nt.html" class="internal">Node Types</a> on metadata associated
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with these.
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</li><li>(b) A node of type <span class="extract"><span class="extract-syntax">A</span></span> can only be a child of a node of type <span class="extract"><span class="extract-syntax">B</span></span> if
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<a href="2-nt.html#SP13" class="internal">NodeType::parentage_allowed</a> says so, and this is (mostly) a matter
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of calling <a href="2-nt.html#SP5" class="internal">NodeType::allow_parentage_for_categories</a> — parentage depends
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not on the type per se, but on the category of the type, which groups types
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together.
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</li><li>(c) A node of type <span class="extract"><span class="extract-syntax">A</span></span> can only have an annotation with ID <span class="extract"><span class="extract-syntax">I</span></span> if
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<a href="2-na.html#SP15" class="internal">Annotations::is_allowed</a> says so. To declare an annotation legal,
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call <span class="extract"><span class="extract-syntax">Annotations::allow(A, I)</span></span>, or <span class="extract"><span class="extract-syntax">Annotations::allow_for_category(C, I)</span></span>
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for the category <span class="extract"><span class="extract-syntax">C</span></span> of <span class="extract"><span class="extract-syntax">A</span></span>.
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</li></ul>
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