/*
s([on(3,1)],P).
P = 0.0625
sc([on(3,1)],[on(2,1)],P).
P = 0.5 ?
sc([on(4,1)],[on(2,1)],P).
P = 0.25
sc([on(10,1)],[on(2,1)],P).
P = 0.00390625
*/
on(0,1):0.5;on(0,2):0.5.
on(X,1):0.5;on(X,2):0.5:-
X1 is X-1,X1>=0,
on(X1,Y),
\+ on(X1,2).
