Provide a test example for '---' in a code fence.

Author: Evan Anderson

spec.txt

@@ -2237,6 +2237,26 @@ aaa
 ````````````````````````````````
 
 
+Within a code fence, other block-level constructs such as lists or thematic
+breaks are ignored.
+
+```````````````````````````````` example
+yaml:
+- A
+- yaml
+- list
+---
+another: "yaml doc"
+.
+<pre><code>yaml:
+- A
+- yaml
+- list
+---
+another: "yaml doc"</code></pre>
+````````````````````````````````
+
+
 Fenced code blocks can interrupt paragraphs, and can be followed
 directly by paragraphs, without a blank line between: