code: @Cached
      @DontLabel
      public Rule unicodeChar(final int codePoint)
      {
          if (!Character.isValidCodePoint(codePoint))
              throw new InvalidGrammarException("invalid code point "
                  + codePoint);
          return new CodePointMatcher(codePoint);
      }
docs: Match one Unicode character
  
  @param codePoint the code point
  @return a rule