All possible combinations of list elements.hs

-- Dumb way (list monad):

combine = do
        a <- list1
        b <- list2
        c <- list3
        return [a, b, c]

-- OK way (list monad):

combinations xs = mapM (>>=) ....  ?????

-- Right way (list comprehensions):

combinations [] = [[]]
combinations (xs:xss) = [ x:y | x <- xs, y <- combinations xss ]