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
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  --------------------------------------------------------------------------------
  
  import Text.Printf
  
  -- | Summing a list of integers.
  add :: (Integral a) => [a] -> a
  add []     = 0
  add (x:xs) = x + add xs
  
  -- | Add together all numbers in a list.
  main = do
      let a = [] :: [Integer]
      let b = [1, 2, 3] :: [Integer]
      let c = [3, 5, 7, 11, 13] :: [Integer]
      let d = [1,4 .. 100] :: [Integer]
      printf "%d: %s\n" (add a) $ show $ sum a == add a
      printf "%d: %s\n" (add b) $ show $ sum b == add b
      printf "%d: %s\n" (add c) $ show $ sum c == add c
      printf "%d: %s\n" (add d) $ show $ sum d == add d
  