Allow terms mapping to null to be protected.

Author: gkellogg

index.html

@@ -1334,15 +1334,9 @@ <h3>Algorithm</h3>
           <a>term definition</a> for <var>term</var> in <var>active context</var>.</li>
         <li>Otherwise, remove any <var>previous definition</var> from
           <var>active context</var>.</li>
-        <li>If <var>value</var> is <code>null</code> or <var>value</var>
-          is a <a class="changed">dictionary</a> containing the key-value pair
-          <code>@id</code>-<code>null</code>, set the
-          <a>term definition</a> in <var>active context</var> to
-          <code>null</code>,
-          and set the value associated with <var>defined</var>'s
-          <a>member</a> <var>term</var> to <code>true</code>, and return.
-          <div class="ednote">Should probably store as a dictionary with <code>@id</code>-<code>null</code>
-            and retain <code>@protected</code> so we can protect against redefinitions.</div></li>
+        <li class="changed">If <var>value</var> is <code>null</code>,
+          convert it to a <a class="changed">dictionary</a> consisting of a single <a>member</a> whose
+          key is <code>@id</code> and whose value is <code>null</code>.</li>
         <li>Otherwise, if <var>value</var> is a <a>string</a>, convert it
           to a <a class="changed">dictionary</a> consisting of a single <a>member</a> whose
           key is <code>@id</code> and whose value is <var>value</var>.
@@ -1414,7 +1408,10 @@ <h3>Algorithm</h3>
         <li>If <var>value</var> contains the <a>member</a> <code>@id</code> and its value
           does not equal <var>term</var>:
           <ol>
-            <li>If the value associated with the <code>@id</code> <a>member</a> is not a <a>string</a>, an
+            <li>If <var>value</var> contains the <code>@id</code> <a>member</a>
+              is <code>null</code>, the term is not used for IRI expansion, but is
+              retained to be able to detect future redefinitions of this term.</li>
+            <li>Otherwise, if the value associated with the <code>@id</code> <a>member</a> is not a <a>string</a>, an
               <a data-link-for="JsonLdErrorCode">invalid IRI mapping</a>
               error has been detected and processing is aborted.</li>
             <li>Otherwise, set the <a>IRI mapping</a> of <var>definition</var> to the
@@ -1697,9 +1694,10 @@ <h3>Algorithm</h3>
               in <var>active context</var> during
               <a href="#context-processing-algorithm">Context Processing</a>.</li>
             <li>If <var>active context</var> contains a <a>term definition</a>
-              for <var>prefix</var>, return the result of concatenating
-              the <a>IRI mapping</a> associated with <var>prefix</var> and
-              <var>suffix</var>.</li>
+              for <var>prefix</var>
+              <span class="changed">having a non-<code>null</code> <a>IRI mapping</a></span>,
+              return the result of concatenating the <a>IRI mapping</a>
+              associated with <var>prefix</var> and <var>suffix</var>.</li>
             <li><span class="changed">If <var>value</var> has the form of an <a>absolute IRI</a></span>,
               return <var>value</var>.</li>
           </ol>

tests/expand-manifest.jsonld

@@ -2338,6 +2338,14 @@
       "option": {"specVersion": "json-ld-1.1"},
       "input": "expand/pr27-in.jsonld",
       "expect": "expand/pr27-out.jsonld"
+    }, {
+      "@id": "#tpr28",
+      "@type": ["jld:NegativeEvaluationTest", "jld:ExpandTest"],
+      "name": "Fails if trying to redefine a protected null term.",
+      "purpose": "A protected term with a null IRI mapping cannot be redefined.",
+      "option": {"specVersion": "json-ld-1.1"},
+      "input": "expand/pr28-in.jsonld",
+      "expect": "protected term redefinition"
     }, {
       "@id": "#ttn01",
       "@type": ["jld:NegativeEvaluationTest", "jld:ExpandTest"],

tests/expand/pr28-in.jsonld

@@ -0,0 +1,11 @@
+{
+  "@context": [{
+    "@version": 1.1,
+    "@protected": true,
+    "term": null
+  }, {
+    "@version": 1.1,
+    "term": {"@id": "http://example.com/term"}
+  }],
+  "term": "undefined"
+}