i j
x s x t x s t x s t
s t
x s x t x s t x t x t
s t
x r x s x t x r s t x r s t
r s t
a b c d
a b
c d
a c b d
a c b d
a b
c d
n n n n
a b a b
a b a b
a b a b
a b a b
a b
a d ad bc a d ad bc
c d
a ad ad bc ab bd a bc ab bd
ac cd ad d ad bc ac cd bc d
a b a b
c d c d
c c c c c c
