private static string? CharToWord(char digit) =>
            digit switch
            {
                '-' => " minus",
                '+' => " plus",
                '.' => " point",
                'E' => " E",
                '0' => " zero",
                '1' => " one",
                '2' => " two",
                '3' => " three",
                '4' => " four",
                '5' => " five",
                '6' => " six",
                '7' => " seven",
                '8' => " eight",
                '9' => " nine",
                _ => null
            };