>>> concat (Just [1, 2, 3]) [1,2,3]
>>> concat (Left 42) []
>>> concat [[1, 2, 3], [4, 5], [6], []] [1,2,3,4,5,6]
>>> concat [] [] >>> concat [[42]] [42] >>> concat [[1,2,3], [4,5], [6], []] [1,2,3,4,5,6]
unstream . concat . fmap stream = concat
toList (concat xss) = concat (map toList xss)
>>> concat [] ""
>>> concat ["foo","bar","doo"] "foobardoo"