delta(st(A,B),st(0,B)) :- A > 0.
delta(st(A,B),st(A,0)) :- B > 0.
delta(st(A,B),st(3,B)) :- A < 3.
delta(st(A,B),st(A,6)) :- B < 6.
delta(st(A,B),st(NA,NB)) :- AB is A+B,
 			    AB6 is AB-6,
			    NA is max(0,AB6),
			    NB is min(AB,6).
delta(st(A,B),st(NA,NB)) :- AB is A+B,
 			    AB3 is AB-3,
			    NB is max(0,AB3),
			    NA is min(AB,3).

delta(st(A0,B0),st(A1,B1)) :- delta(st(A0,B0),st(A2,B2)), write(st(A2,B2)),
                              delta(st(A2,B2),st(A1,B1)).