regexp :: Int -> Gen (Regexp Char)
regexp 0 = frequency [ (1, return one)
                     , (8, tok <$> (singleton <$> elements "abcdef"))
                     ]
regexp :: Int -> Gen (Regexp Char)
regexp 0 = frequency [ (1, return one)
                     , (8, tok <$> (singleton <$> elements "abcdef"))
                     ]
regexp 0 = frequency [ (1, return one)
                     , (8, tok <$> (singleton <$> elements "abcdef"))
                     ]