(CONDITIONTYPE ORIENTED)
(VAR x)
(RULES
  even(0) -> true
  even(s(x)) -> false | odd(x) == false
  even(s(x)) -> true | odd(x) == true
  odd(0) -> false
  odd(s(x)) -> false | even(x) == false
  odd(s(x)) -> true | even(x) == true
)
(COMMENT
  \cite{R05}, Example 1, p. 10
  doi: 10.1007/978-3-540-31959-7_13
)