code: function(o) {
        if (_.isEmpty(o)) return '';
        var pilimit = o.pilimit || 1;
        pilimit = (pilimit > 50) ? 50 : (pilimit < 1 ? 1 : pilimit);
        var pithumbsize = o.pithumbsize || 50;
        return '&pilimit=' + pilimit + '&pithumbsize=' + pithumbsize;
      }
docs: /*
  Generate part of url to get thumbnails of pages using parameters in *o*
  
  Parameter *o*, required, defined as {
  pilimit, required, int, not bigger than 50, 1 by default,
  pithumbsize, required, int, 50 by default,
  }
  
  Will generate api GET url for prop=pageimages, using following default
  parameters:
  pilimit, no more than 50,
  pithumbsize, 50 by default,
  piprop, no use,
  picontinue, no use.
  }
  
  For more details, check http://home.huiji.wiki/api.php prop=pageimages
  section
  
  Return query string used by prop=pageimages, e.g.,
  &pilimit=20&pithumbsize=320