import "Prelude"
main = do
    print $ [1,2] \\ []
    print $ [1,2,3] \\ [1]
    print $ [1,2,3] \\ [2]
    print $ [1,2,3] \\ [2,3]
--
[1, 2]
[2, 3]
[1, 3]
[1]
()    
--
import "Prelude"

assertEquals :: Show a => a -> a -> <Proc> ()
assertEquals a b = if a == b then () else fail "\(a) <> \(b)"

testGrouping outVal f l = do
    kl = map (\v -> (f v, v)) l
    groups = group kl
    print groups
    
    assertEquals groups (groupBy f l)
    
    i1 = fromMaybe [] . index groups
    i2 = indexGroup kl
    i3 = indexGroupBy f l
    
    keys = map fst groups + [outVal]
    
    for keys $ \k -> do 
        assertEquals (i1 k) (i2 k)
        assertEquals (i1 k) (i3 k)

main = do
    testGrouping 4 (`mod` 3) [2,4,6,8,10]
--
[(2, [2, 8]), (1, [4, 10]), (0, [6])]
()