(CONDITIONTYPE ORIENTED)
(VAR x)
(RULES
  even(0) -> true
  even(s(x)) -> true | odd(x) == true
  odd(s(x)) -> true | even(x) == true
)
(COMMENT
  \cite{NOF13}, example 2.2
  http://www.iaeng.org/IJCS/issues_v40/issue_2/IJCS_40_2_08.pdf
)