}
      checkState(!hasUpperCase(), "Cannot call upperCase() on a mixed-case alphabet");
      char[] upperCased = new char[chars.length];
      for (int i = 0; i < chars.length; i++) {
        upperCased[i] = Ascii.toUpperCase(chars[i]);
      }
      Alphabet upperCase = new Alphabet(name + ".upperCase()", upperCased);
      return ignoreCase ? upperCase.ignoreCase() : upperCase;
    }

    Alphabet lowerCase() {