(VAR x)
(RULES
  b -> a
  b -> c
  c -> c
  d -> c
  d -> e
  f(x, a) -> A
  f(x, e) -> A
  f(x, A) -> A  
  f(c, x) -> A
)
(COMMENT p.30 of \cite{IWC16}
  GUN & ~UN & ~GUNC
)