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.

2012/tromp/README.md

@@ -520,11 +520,6 @@ The program has been tested to work correctly on Linux/Solaris/MacOSX both in 32
 and 64 bits.
 
 
-### How the program works
-
-See the file [how.html](how.html).
-
-
 ### Acknowledgements
 
 Christopher Hendrie, Bertram Felgenhauer, Alex Stangl, Seong-hoon Kang,
@@ -547,6 +542,19 @@ Higher Order Symbol. Comput. 20, 3 (September 2007), 199-207.
 <http://www.pps.univ-paris-diderot.fr/~krivine/articles/lazymach.pdf>
 
 
+### 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.
+
+
+### How this entry works:
+
+See the file [how.html](how.html).
+
+
 <!--
 
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