Start on distinct types

Part of #413.

lang/spec.html

@@ -584,7 +584,7 @@ <h4>Type descriptors</h4>
    <td>a sequence of values that can be generated lazily</td>
   </tr>
   <tr>
-   <td rowspan="9">other</td>
+   <td rowspan="11">other</td>
    <td>singleton</td>
    <td>a single value described by a literal</td>
   </tr>
@@ -612,6 +612,10 @@ <h4>Type descriptors</h4>
    <td>intersection</td>
    <td>a value that belongs to all of a number of types</td>
   </tr>
+  <tr>
+   <td>distinct</td>
+   <td></td>
+  </tr>
   <tr>
    <td>anydata</td>
    <td>plain data (a simple value, sequence value or structured value that
@@ -2202,6 +2206,7 @@ <h3>Other type descriptors</h3>
    | optional-type-descriptor
    | union-type-descriptor
    | intersection-type-descriptor
+   | distinct-type-descriptor
    | anydata-type-descriptor
    | json-type-descriptor
    | byte-type-descriptor
@@ -2361,6 +2366,15 @@ <h4>Intersection types</h4>
 <code>readonly&amp;T</code>, where <code>T</code> describes the primary aspect
 of the shape.
 </p>
+</section>
+
+<section>
+<h4>Distinct types</h4>
+
+<pre
+class="grammar">distinct-type-descriptor := <code>distinct</code> type-descriptor
+</pre>
+
 </section>
 <section>
 <h4 id="anydata">Anydata type</h4>
@@ -7914,6 +7928,8 @@ <h3>Summary of changes from 2020R1 to 2020R2</h3>
 <ol>
 <li>The <code>readonly</code> type has been added.</li>
 <li>Intersection types have been added.</li>
+<li>Distinct types have been added; these provide the functionality
+of nominal types with the framework of a structural type system.</li>
 <li>The table type has been redesigned to be more consistent with other
 structural types and no longer has preview status.</li>
 <li>Enum declarations have been added.</li>