Date: Mon, 13 Jul 81 11:33:00 -0700 (PDT)
~~~ From: Bill McKeeman <McKeeman@PARC-MAXC >

Have you seen the Missing Link? ["By the people who brought you Rubik's
Cube"; oh well, if they say so] It is a cylindrical 15-puzzle, and about as hard
to solve. Here are some notes on it.

The actual puzzle is a square tower of height four. When solved, the puzzle
has the appearance of linked chains. It has 15 square pieces leaving one hole
through which you can peek to see the innards of the puzzle. The hole can
move up or down by sliding the piece next to it down or up. The top and
bottom slices rotate about the vertical axis which permits the hole to move to
different columns. A rough idea of the pattern is:

```			|~|n|  can rotate
edge-on view	|n|X|
|X|X|
|u|u|  can rotate
```

I use two transformations, r and R to do things with it. If you look at an edge of
the puzzle with the hole showing, there is a little cycle and a big cycle as noted
below. Both leave the back unchanged.

transformation from: to:
name

```r		|::|1|     	|::|2|
|7|2|  =>	|1|7|
|6|3|     	|6|3|
|5|4|     	|5|4|

R		|::|1|     	|1|2|
|7|2| =>	|::|3|
|6|3|     	|7|4|
|5|4|     	|6|5|
```

The move set is:
U move the hole up
D move the hole down
T twist the top clockwise 90 degrees
B twist the bottom clockwise 90 degrees

for convenience, /T = TTT = T inverse, etc.

```r = D TU/T D /TUT
RR = /TDT DD /BUB U TU/T DD BD/B U
```

Actually, I can "feel" my way to a solution easier than I can figure my way to
one. It feels very much like the 15 puzzle.