def characters
      chars = raw_characters
      extended ? chars.map{|c| EXTENDED_ENCODINGS[c].split(//) }.flatten : chars
    end
def characters
      chars = raw_characters
      extended ? chars.map{|c| EXTENDED_ENCODINGS[c].split(//) }.flatten : chars
    end
Returns the encodable characters. If extended mode is enabled, each character will
first be replaced by two characters from the encodable charset