Define how "Assert:" works

See #87 for direct inspiration, though we’ve been using this here and
there for a while now.

infra.bs

@@ -203,6 +203,18 @@ iteration.
 </div>
 
 
+<h3 id=assertions>Assertions</h3>
+
+<p>To improve readability, it can sometimes help to add assertions to algorithms, stating
+invariants. To do this, write "<code>Assert: </code>", followed by a statement that must be true.
+
+<div class=example id=example-assert>
+ <ol>
+  <li><p>Let |x| be "<code>Aperture Science</code>".
+  <li><p>Assert: |x| is "<code>Aperture Science</code>".
+</div>
+
+
 <h2 id=primitive-data-types>Primitive data types</h2>
 
 <h3 id=bytes>Bytes</h3>