2 | 6. c: T List 7. u1: T 8. v1: T List 9. b,d:T List.
[u / (v @ b)] = (v1 @ d)

( e:T List. [u / v] = (v1 @ e) & d = (e @ b) v1 = [u / (v @ e)] & b = (e @ d)) b,d:T List. [u / (v @ b)] = [u1 / (v1 @ d)]  ( e:T List. [u / v] = [u1 / (v1 @ e)] & d = (e @ b) [u1 / v1] = [u / (v @ e)] & b = (e @ d)) | 1 step |