(VAR x)
(RULES
  b(w(x)) -> w(w(w(b(x))))
  w(b(x)) -> b(x)
  b(b(x)) -> w(w(w(w(x))))
  w(w(x)) -> w(x)
)
(COMMENT
doi:10.1007/978-1-4757-3661-8
[18] Exercise 4.2.2
submitted by: Takahito Aoto, Junichi Yoshida, and Yoshihito Toyama
)