code: public static Parser<Void> nestableBlockComment(
        Parser<?> begin, Parser<?> end, Parser<?> commented) {
      return new NestableBlockCommentScanner(begin, end, commented);
    }
docs: A scanner for a nestable block comment that starts with {@code begin} and ends with
  {@code end}.
  
  @param begin starts a block comment
  @param end ends a block comment
  @param commented the commented pattern except for nested comments.
  @return the block comment scanner.