[0 -> (1 (\ 2))](0 (\ 1))
= [0 -> (1 (\ 2))]0 [0 -> (1 (\ 2))](\ 1))
= (1 (\ 2)) (\ [1 -> (2 (\ 3))]1))
= (1 (\ 2)) (\ (2 (\ 3)))

[0 -> 1] (0 (\ 1 0))
= [0 -> 1]0 [0 -> 1](\ 1 0))
= 1 (\ [1 -> 2](1 0)))
= 1 (\ [1 -> 2]1 [1 -> 2]0))
= 1 (\ 2 0)