[Bug-gnubg] Confusing analysis of Too Good decision

[Ian Shaw]

    GNU Backgammon  Position ID: tm0BBByz7wYAAA
                    Match ID   : UQmgAAAAEAAA
    +24-23-22-21-20-19------18-17-16-15-14-13-+  O: tasmanian_devil
    |    O  O  O  O  O |   | O                |  0 points
    |    O  O  O  O  O |   |                  |  
    |                  |   |                  |  
    |                  |   |                  |  
    |                  |   |                  |  
    |                  |BAR|                  |v 5 point match
    |          X       |   |                  |  
    |          X       |   |                  |  
    |    O     X  X    |   |                  |  
    | X  O  X  X  X  X |   |                  |  On roll
  X | X  O  X  X  X  X |   |       O          |  2 points
    +-1--2--3--4--5--6-------7--8--9-10-11-12-+  X: ian (Cube: 2)

I'm confused by GnuBg's analysis of this position. 

The annotation window says No Redouble, Take.
The game analysis reports this as a Wrong Double around Too Good, instead of
Wrong Double around Doubling Point
The Market Window display has it Too Good and Not Good Enough, if I
interpret it correctly.
I think it is No Redouble, Take.

Annotation Window:

Cube analysis
2-ply cubeless equity  +1.007
  0.816 0.436 0.017 - 0.184 0.000 0.000
1. No double            +0.951
2. Double, pass         +1.000  ( +0.049)
3. Double, take         +0.808  ( -0.143)
Proper cube action: No redouble, take (74.6%)

Game Analysis window:
                                      tasmanian_devil      ian

Wrong doubles around DP               1 (-0.187, -1.494%)  0 (+0.000,
+0.000%) 
Wrong doubles around TG               0 (+0.000, +0.000%)  1 (-0.143,
-2.511%) 

Market Window erm... Window:)

The Market Window display shows my Too Good Point to be 83.871%, which is
more than the 81.6% indicated by the analysis. So according to this I am not
Too Good.
The opponents Take Point is shown as 20.000%. Since he only has 18.4%, this
would indicate a Pass. However, I am using the Woolsey-Heinrich MET. O can
take and recube for the match, or drop to 5-away 1-away Crawford, which is
15%. With 18.4% chances O's correct cube action is to Take, as indicated by
the analysis. The Market Window must be wrong.

GnuBg actually thinks this is one of those "Too Good and Not Good Enough"
positions. It has the Doubling Point at 88.235% and the Cash Point at
85.000%. Aha! The opponents Take Point of 20.000% doesn't take the recube
into account, but my Cash Point lists both. These are surely one and the
same thing, viewed from opposite sides of the board.

FWIW, my calculations follow.
X doesn't cube:
X wins single game to go 1aC 5a         =  85%
X wins gammon for match                 = 100%
43.6/81.6 = 53.4% of my X's wins are gammons so pro-rated when I win I get
92.0% MWC
O wins game to go 3a 3a                 = 50%

X cubes and O redoubles:
X wins match 81.6%
O wins match 18.4%

By cubing, X risks 50% to gain 10.4%, so the doubling window opens at 82.78%
(GnuBg has it at 88.235%).

By this calculation the correct action is a straightforward No Redouble,
Take. This agrees with the annotation window but not with the game analysis,
which lists it as a "Wrong double around TG" rather than a "Wrong double
around DP" (see above).

Can anyone make sense of all this?

Yours,
Confused of Sheffield