-- Problems with the use of backtracking for search.

data AB = A | B
 deriving Show

p :: AB -> Bool
p A = True
p B = True

q :: AB -> Bool
q B = True

r :: AB -> Bool
r A = r A
r B = True

-- The evaluation of the expression
--
--     r x where x free
--
-- does not terminate in backtracking-based implementations, e.g.,
-- in [PAKCS](https://www.informatik.uni-kiel.de/~pakcs/)
--
-- However, it does terminate in an operational complete implementation, e.g.,
-- in [Curry2Go](https://www-ps.informatik.uni-kiel.de/curry2go/)