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.

2019/duble/README.md

@@ -184,7 +184,7 @@ You will probably have a hard time analyzing this entry, because:
 ### A few more things:
 
 Did you notice the `enum` declaration? Could it be just an `int` or `const int`
-variable, instead? (spoiler: <http://c-faq.com/ansi/constasconst.html>)
+variable, instead?
 
 If your keyboard has no arrow keys, you can probably find alternate keys by
 reading the source code.
@@ -250,6 +250,19 @@ maybe not reduce consumption, but consume them differently...  And I obtained
 it works on macOS too!  I must confess it was challenging to remain below the
 size limit with this little change.
 
+
+### NOTICE to those who wish for a greater challenge
+
+**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.
+
+
+### For more information
+
+See <http://c-faq.com/ansi/constasconst.html> for more information.
+
 <!--
 
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