add notices to those who wish for a greater challenge

We add the following level-3 header to help people identify text
they may wish to skip if they wish for a greater challenge:

```
```

After that notice we add:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.
```

And sometimes follow with:

```
```

Or sometimes follow with:

```
```

Where the alt code impacts a "greater challenge", we add instead:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program without looking at the Alternate code.

If you get stuck, come back and look at the Alternate code.
```

And where a given command impacts a "greater challenge", we add instead:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program without running the command below.

If you get stuck, come back and run the following command:
```

Perform `make www` to perform the above and test under macOS.

1998/dorssel/index.html

@@ -466,11 +466,19 @@ text that can be reversibly Morsified, i.e.</li>
 <pre><code>    ./dorssel &lt; dorssel.c | ./dorssel | ./dorssel | ./dorssel</code></pre>
 <p>is the same as</p>
 <pre><code>    ./dorssel &lt; dorssel.c | ./dorssel</code></pre>
+<h3 id="notice-to-those-who-wish-for-a-greater-challenge">NOTICE to those who wish for a greater challenge</h3>
+<p><strong>If you want a greater challenge, don’t read any further</strong>:
+just try to understand the program without running the command below.</p>
+<p>If you get stuck, come back and run the following command:</p>
+<pre><code>    ./dorssel &lt; dorssel.md</code></pre>
+<h3 id="notice-to-those-who-wish-for-a-greater-challenge-1">NOTICE to those who wish for a greater challenge</h3>
+<p><strong>If you want a greater challenge, don’t read any further</strong>:
+just try to understand the program via the source.</p>
+<p>If you get stuck, come back and read below for additional hints and information.</p>
 <ul>
 <li>The file <a href="dorssel.html">dorssel.html</a> contains all the information needed to
 understand this program. For spoiler information, try:</li>
 </ul>
-<pre><code>    ./dorssel &lt; dorssel.md</code></pre>
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