content: --------------------------------------------------------------------------------
  -- MIT License
  --
  -- Copyright (C) 2023 Duck McSouls <quacksouls [AT] gmail [DOT] com>
  --
  -- Permission is hereby granted, free of charge, to any person obtaining a copy
  -- of this software and associated documentation files (the "Software"), to deal
  -- in the Software without restriction, including without limitation the rights
  -- to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  -- copies of the Software, and to permit persons to whom the Software is
  -- furnished to do so, subject to the following conditions:
  --
  -- The above copyright notice and this permission notice shall be included in
  -- all copies or substantial portions of the Software.
  --
  -- THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  -- IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  -- FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  -- AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  -- LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  -- OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  -- SOFTWARE.
  --------------------------------------------------------------------------------
  
  import Text.Printf
  
  -- | Concatenate a list of lists.  This is the same as flattening the list.
  flatten :: [[a]] -> [a]
  flatten []     = []
  flatten (x:xs) = x ++ flatten xs
  
  -- | List concatenation.
  main = do
      let a = [] :: [[Integer]]
      let b = [[1, 2, 3]]
      let c = [[1 .. 5], [2,4 .. 10], [3,5 .. 9]]
      let d = ["abc", "def", "g"]
      let e = [[True, False], [False, False], [True, True]] :: [[Bool]]
      printf "%s -> %s\n" (show $ flatten a) $ show $ flatten a == concat a
      printf "%s -> %s\n" (show $ flatten b) $ show $ flatten b == concat b
      printf "%s -> %s\n" (show $ flatten c) $ show $ flatten c == concat c
      printf "%s -> %s\n" (show $ flatten d) $ show $ flatten d == concat d
      printf "%s -> %s\n" (show $ flatten e) $ show $ flatten e == concat e
  