/*
s([on(0,1)],P).
P = 0.333333333333333 ?

s([on(1,1)],P).
P = 0.222222222222222 
s([on(2,1)],P).
P = 0.148148147703704 
s([on(3,1)],P).
% OUT OF DATABASE SPACE ERROR- Database crashed against Stacks
s([on(4,1)],P).
% OUT OF DATABASE SPACE ERROR- Database crashed against Stacks
s([on(5,1)],P).
% OUT OF DATABASE SPACE ERROR- Database crashed against Stacks

s([on(0,1),on(1,1)],P).
P = 0.111111111111111
s([on(0,1),on(1,1),on(2,1)],P).
P = 0.037037037037037
sc([on(1,1)],[on(0,1)],P).
P = 0.333333333333333

sc([on(2,1)],[on(0,1)],P).
P = 0.222222222222222

sc([on(3,1)],[on(0,1)],P).
P = 0.148148148148148

sc([on(2,1)],[on(1,1)],P).
P = 0.333333333333333 ?


*/
on(0,1):1/3;on(0,2):1/3;on(0,3):1/3.

on(X,1):1/3;on(X,2):1/3;on(X,3):1/3:-
  X1 is X-1,X1>=0,
  on(X1,Y),
  \+ on(X1,3).