import Prelude hiding ( not )
import Test.LeanCheck

t1 = check True

not :: Bool -> Bool
not False = True
not True = False

t2 = check (not False)
t3 = check $ \ x -> x == not (not x)

xor :: Bool -> Bool -> Bool
xor False y = y
xor True  y = not y

t4 = check $ \ x y -> xor x  y ==  xor y x