unionAll-naive

unionAll :: (Ord a) => [[a]] -> [a]
unionAll xs =
    let
        xs' = filter (not.null) xs                   -- | filter empty lists, if any
    in                                               
        if null xs'                                  -- | if nothing is left after filtering
        then []                                      -- | return the empty list
        else                                         -- | otherwise
            let                                      
                next = minimum (map head xs')        -- | find the minimum of head elements
                                                     -- | they are minimum values in their respective lists
                                                     -- | so this is the minimum of every list
                                                     
                xs'' = map (dropWhile (next==)) xs'  -- | remove that value from every list
                                                     -- | (no need to filter this time, since lists are ordered)
                                                     
            in next : unionAll xs''                  -- | recursively call unionAll on xs''