docs: public static String decode(String ciphertext, int offset) {
  		final int len = ciphertext.length();
  		final char[] plain = ciphertext.toCharArray();
  		char c;
  		for (int i = 0; i < len; i++) {
  			c = ciphertext.charAt(i);
  			if (false == Character.isLetter(c)) {
  				continue;
  			}
  			plain[i] = decodeChar(c, offset);
  		}
  		return new String(plain);
  	}