public Letters Subtract(Letters second)
{
string letters1 = characters;
string letters2 = second.characters;
string different_letters = "";
while (true)
{
if (letters2.Length == 0) return new Letters(different_letters + letters1);
if (letters1.Length == 0) return null;
{
char letter2 = letters2[0];
char letter1 = letters1[0];
if (letter1 > letter2) return null;
if (letter1 < letter2)
{
letters1 = letters1.Substring(1);
different_letters += letter1;
continue;
}
//if (letter1 == letter2) throw new Exception("Error: letter1 == letter2");
letters1 = letters1.Substring(1);
letters2 = letters2.Substring(1);
}
}
}
private static string Subtract(string first, string sencod)
{
Letters letters1 = new Letters(first);
Letters letters2 = new Letters(sencod);
Letters different_letters = letters1.Subtract(letters2);
if (different_letters == null) return null;
return different_letters.ToString();
}