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.

2018/yang/README.md

@@ -42,15 +42,6 @@ reveal messages and one more tool that can be used to reveal the final message
 hidden in the original source.  All of these can be created using combinations
 of `./left`, `./right` and `./shift` and the additionally generated programs.
 
-The final message can be revealed using
-
-``` <!---sh-->
-    ./msg9 < prog.c
-```
-
-But what combinations will generate `./msg3`, `./msg4`, `./msg5`, `./msg6`,
-`./msg7`, `./msg8` and finally `./msg9`?
-
 
 ## Author's remarks:
 
@@ -169,6 +160,34 @@ will look weird after rotation, for example.
 `Nuko` compiles without warnings with all compilers above, even with
 `-Wall -Wextra -pedantic` for gcc and clang.
 
+
+### 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 without running the command below.
+
+If you get stuck, come back and run the following command:
+
+``` <!---sh-->
+    ./msg9 < prog.c
+```
+
+But what combinations will generate `./msg3`, `./msg4`, `./msg5`, `./msg6`,
+`./msg7`, `./msg8` and finally `./msg9`?
+
+For more hints try:
+
+``` <!---sh-->
+    make details
+```
+
+And for the last word, try:
+
+``` <!---sh-->
+    make the_last_word
+```
+
+
 <!--
 
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