Update fromRdf algorithm to consider list nodes that are in multiple graphs, as considered in json-ld/json-ld.org#357 (comment).

Fixes #13.

Author: gkellogg

index.html

@@ -929,7 +929,7 @@ <h3>Algorithm</h3>
                   <code>@vocab</code> key.</li>
                 <li>If <var>value</var> is <a>null</a>, remove
                   any <a>vocabulary mapping</a> from <var>result</var>.</li>
-                <li class="changed">Otherewise, if <var>value</var>
+                <li class="changed">Otherwise, if <var>value</var>
                   the empty string (<code>&quot;&quot;</code>),
                   the effective value is the current <a>base IRI</a>.</li>
                 <li>Otherwise, if <var>value</var> is
@@ -4022,7 +4022,7 @@ <h2>Algorithm</h2>
         <li>Initialize <var>graph map</var> to a <a class="changed">dictionary</a> consisting
           of a single member <code>@default</code> whose value references
           <var>default graph</var>.</li>
-        <li>Initialize <var>node usage map</var> to an empty <a class="changed">dictionary</a>.</li>
+        <li class="changed">Initialize <var>referenced once</var> to an empty <a>dictionary</a>.</li>
         <li>For each <var>graph</var> in  <var>dataset</var>:
           <ol>
             <li>If <var>graph</var> is the <a>default graph</a>,
@@ -4069,19 +4069,24 @@ <h2>Algorithm</h2>
                   associated with the <var>predicate</var> member of <var>node</var>, append a
                   reference to <var>value</var> to the <a>array</a>. Two JSON objects
                   are considered equal if they have equivalent key-value pairs.</li>
-                <li>If <var>object</var> is a <a>blank node identifier</a> or <a>IRI</a>,
-                  it might represent the list node:
+                <li class="changed">If <var>object</var> is <code>rdf:nil</code>, it represents
+                  the termination of an <a>RDF collection</a>:
                   <ol>
-                    <li>If the <var>object</var> member of <var>node usage map</var> does not exist,
-                      initialize it to a new empty <a>array</a>.</li>
-                    <li>Append the value of the <code>@id</code> member of <code>node</code> to
-                      the <var>object</var> member of <var>node usage map</var>.</li>
-                    <li>If the <var>object</var> member of <var>node map</var> has no
-                      <code>usages</code> member, create one and initialize it to
-                      an empty <a>array</a>.</li>
                     <li>Reference the <code>usages</code> member of the <var>object</var>
                       member of <var>node map</var> using the variable <var>usages</var>.</li>
-                    <li>Append a new <a class="changed">dictionary</a> consisting of three
+                    <li>Append a new <a>dictionary</a> consisting of three
+                      members, <code>node</code>, <code>property</code>, and <code>value</code>
+                      to the <var>usages</var> <a>array</a>. The <code>node</code> member
+                      is set to a reference to <var>node</var>, <code>property</code> to <var>predicate</var>,
+                      and <code>value</code> to a reference to <var>value</var>.</li>
+                  </ol>
+                </li>
+                <li class="changed">Otherwise, if <var>referenced once</var> has an entry for <var>object</var>,
+                  set the <var>object</var> member of <var>referenced once</var> to <code>false</code>.</li>
+                <li class="changed">Otherwise, if <var>object</var> is a <a>blank node identifier</a>,
+                  it might represent a list node:
+                  <ol>
+                    <li>Set the <var>object</var> member of <var>referenced once</var> to a new <a>dictionary</a> consisting of three
                       members, <code>node</code>, <code>property</code>, and <code>value</code>
                       to the <var>usages</var> <a>array</a>. The <code>node</code> member
                       is set to a reference to <var>node</var>, <code>property</code> to <var>predicate</var>,
@@ -4112,11 +4117,8 @@ <h2>Algorithm</h2>
                 <li>While <var>property</var> equals <code>rdf:rest</code>,
                   <span class="changed">the value of the <code>@id</code> member
                     of <var>node</var> is a <a>blank node identifier</a>,</span>
-                  the <a>array</a> value of the member of <var>node usage map</var> associated with the <code>@id</code>
-                  member of <code>node</code> has only one member,
-                  <span class="changed">
-                    the value associated to the <code>usages</code> member of <i>node</i> has
-                    exactly 1 entry,</span>
+                  the value of the member of <var>referenced once</var> associated with the <code>@id</code>
+                  member of <code>node</code> is a <a>dictionary</a>,
                   <var>node</var> has a <code>rdf:first</code> and <code>rdf:rest</code> property,
                   both of which have as value an <a>array</a> consisting of a single element,
                   and <var>node</var> has no other members apart from an optional <code>@type</code>
@@ -4129,8 +4131,8 @@ <h2>Algorithm</h2>
                       <var>node</var> to the <var>list</var> <a>array</a>.</li>
                     <li>Append the value of the <code>@id</code> member of
                       <var>node</var> to the <var>list nodes</var> <a>array</a>.</li>
-                    <li>Initialize <var>node usage</var> to the only item of the
-                      <code>usages</code> member of <var>node</var>.</li>
+                    <li>Initialize <var>node usage</var> to the value of the member of <var>referenced once</var> associated with the <code>@id</code>
+                      member of <code>node</code>.</li>
                     <li>Set <var>node</var> to the value of the <code>node</code> member
                       of <var>node usage</var>, <var>property</var> to the value of the
                       <code>property</code> member of <var>node usage</var>, and