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