docs: function(block, pos, ctx, callback) {
    if (block.hasOwnProperty('htmlslabs')) {
      outputSlabs(block.htmlslabs, ctx, callback);
    } else {
      if (formats.hasOwnProperty(block.format)) {
        formats[block.format](block, pos, ctx, callback);
      } else {
        callback(new Error('unknown format: ' + block.format));
      }
    }
  }