--  [1, 2, 3, 4, 5]                                => normal form
--  1 : 2 : 3 : 4 : _                              => weak normal form
-- enumFromTo 1 10                                 => neither
-- length [1, 2, 3, 4, 5]                          => neither
-- sum (enumFromTo 1 10)                           => neither
-- ['a'..'m'] ++ ['n'..'z']                        => weak normal form
-- (_, 'b')                                        => normal form