1. black(X1) =. true :- crl(X1) =. true,next(X2,X1) =. true,next(X3,X2) =. true.

2. white(X1) =. true :- sqr(X1) =. true,next(X1,X2) =. true,crl(X2) =. true.

3. shaded(X1) =. true :- first(X1) =. true,sqr(X1) =. true.

4. shaded(X1) =. true :- next(X1,X2) =. true,shaded(X2) =. true.

5. black(X1) =. true :- crl(X1) =. true,next(X1,X2) =. true,white(X2) =. true.

6. white(X1) =. true :- sqr(X1) =. true,next(X1,X2) =. true,white(X2) =. true.