The Game:
The game is played on a 'board' with seven positions, 1 to 7.
A marker 'o' is placed on the centre position, 4, and the end positions, 1 
and 7 are goals, marked 'X' and 'Y':

	<X>  < >  < >  <o>  < >  < >  <Y>
	 1    2    3    4    5    6    7

The two players, Px and Py each start with a pool of 50 units.
Each turn then takes the following form:

1. each player selects a number of their remaining units to stake:
   this may be any number from zero units to all the units they have left
2. when both players have decided how much to stake, these amounts are revealed
   and the marker is moved according to who staked most:
   If Px staked more units then the marker is moved one position closer to
   his opponent's goal Y.
   Similarly, if Py staked more then the marker is moved toward X.
   If both players staked the same amount then the marker is not moved.
3. The amounts each player staked are removed from their pool of units.

The game is won by Px if the marker is ever moved into position Y
and Py wins if the marker ever gets to X.
A draw may occur if both players run out of units while the marker is
still on one of the positions 2 to 6 (this is rare in practice).

The Puzzle:
The puzzle is to define the circumstances under which either player can
be said to have a forced win.

For instance, Px clearly has a forced win if the marker is on position 6
and Px has more units than Py (Px wins by staking all his remaining units).