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unionAll :: (Ord a) => [[a]] -> [a]
unionAll xs =
xs' = filter (not.null) xs -- | filter empty lists, if any
if null xs' -- | if nothing is left after filtering
then [] -- | return the empty list
else -- | otherwise
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''
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