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-- Half of a natural number:
import Test.EasyCheck
data Peano = O | S Peano
toPeano :: Int -> Peano
toPeano n = if n==0 then O else S (toPeano (n-1))
fromPeano :: Peano -> Int
fromPeano O = 0
fromPeano (S p) = 1 + fromPeano p
add :: Peano -> Peano -> Peano
add O p = p
add (S p) q = S (add p q)
half :: Peano -> Peano
half y | add x x == y
= x
where x free
main :: Int
main = fromPeano (half (toPeano 100))
test_half = fromPeano (half (toPeano 100)) -=- 50
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