/*
   ?- s([win(white)],P).
P = 0.5 ?
yes
   ?- s([win(black)],P).
P = 0.5 ?
yes
   ?- s([win(black),win(white)],P).
P = 0 ?
*/

win(white) :- \+ win(black).
win(black):- \+ win(white).
win(white) : 0.5 ; win(black) : 0.5.