/*

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,_),
  \+ on(X1,2).