code: function(walk, openMarker, closeMarker, emitter) {
      if (!walk.at(openMarker))
        return false;
      walk.skip(openMarker.length);
      var end = walk.indexOf(closeMarker);
      if (end === null) {
        // just emit the open marker
        this.out.push(openMarker);
      } else {
        // delegate rendering of subarea to emitter
        var w = new SubWalker(walk, walk.position, end);
        emitter.call(this, w);
        walk.startFrom(end + closeMarker.length);
      }
      return true;
    }
docs: /* Bracing elements -- code spans, ems and strongs -- are processed
  like logical parentheses.