Modify the path by adding another path.
(both </>~ "bin" $ ("hello","world")) == ("hello" </> "bin", "world" </> "bin")True
(</>~) :: Setter s a FilePath FilePath -> FilePath -> s -> a
(</>~) :: Iso s a FilePath FilePath -> FilePath -> s -> a
(</>~) :: Lens s a FilePath FilePath -> FilePath -> s -> a
(</>~) :: Traversal s a FilePath FilePath -> FilePath -> s -> a