[Bug-gnubg] Too good to double when a gammon is impossible?!

[Timothy Y. Chow]

I just rolled out a cube decision and gnubg evaluated "no double" at 
greater than +1.000 equity despite the fact that a gammon was no longer 
possible.  Below is the text output; I can email the .sgf file if anyone
wants it.

Tim


The score (after 0 games) is: gnubg 0, tchow 0
Match Information:

Date: 08/21/2009
Move number 1:  gnubg on roll, cube decision?

 GNU Backgammon  Position ID: bgAAULZtEwAiAA
                 Match ID   : AQEAAAAAAAAA
 +12-11-10--9--8--7-------6--5--4--3--2--1-+     O: gnubg (Cube: 2)
 |          O     O |   | O  O  O  O  O  X |     0 points
 |                O |   | O  O  O  O  O    |     On roll
 |                  |   |                  |
 |                  |   |                  |
 |                  |   |                  |
^|                  |BAR|                  |
 |                  |   |                  | X
 |                  |   |                  | X
 |                  |   |             X    | XX
 |                  |   |          X  X    | XX
 |                  | X |       O  X  X  O | XX  0 points
 +13-14-15-16-17-18------19-20-21-22-23-24-+     X: tchow
Pip counts: O 108, X 61

* gnubg doubles

Alert: wrong double ( -0.167)!
Rollout details:
Player gnubg owns 2-cube:
  0.828 0.000 0.000 - 0.172 0.007 0.001 CL  +0.647 CF  +1.167
 [0.002 0.000 0.000 - 0.002 0.000 0.000 CL   0.004 CF   0.010]
Player tchow owns 4-cube:
  0.859 0.000 0.000 - 0.141 0.006 0.001 CL  +1.422 CF  +1.325
 [0.002 0.000 0.000 - 0.002 0.000 0.000 CL   0.008 CF   0.010]
Full cubeful rollout with var.redn.
1296 games, Mersenne Twister dice gen. with seed 866958217 and 
quasi-random dice
Play: 0-ply cubeful [expert]
Cube: 0-ply cubeful [expert]