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
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  --------------------------------------------------------------------------------
  
  import Text.Printf
  
  -- | Cartesian product of lists.
  cartprod :: [a] -> [b] -> [(a, b)]
  cartprod [] [] = []
  cartprod _ [] = []
  cartprod [] _ = []
  cartprod (a:b:c) (x:y:z) =
      [ (a, x)
      , (a, y)
      , (a, head z)
      , (b, x)
      , (b, y)
      , (b, head z)
      , (head c, x)
      , (head c, y)
      , (head c, head z)
      ]
  
  -- | An implementation of Cartesian product.
  main = do
      let a = []
      let b = [1, 2, 3]
      let c = "abc"
      printf "[] x [] -> %s\n" $ show $ length (cartprod a a) == 0
      printf "[] x %s-> %s\n" (show b) $ show $ length (cartprod a b) == 0
      printf "%s x []-> %s\n" (show c) $ show $ length (cartprod c a) == 0
      printf "%s x %s -> %s\n" (show b) (show c) $ show $ cartprod b c
  