docs: def bulletins(self, rt="", rtdir="", stpid=""):
          """
          Return list of service alerts ('bulletins') for a route or stop.
          
          Arguments:
              `rt`: route designator
              or
              `stpid`: bus stop number
              or (`rt` and `rtdir`) or (`rt` and `rtdir` and `stpid`)
          
          Response:
               `sb`: (bulletin container) contains list of
                   `nm`: bulletin name/ID
                   `sbj`: bulletin subject
                   `dtl`: full text and/or
                   `brf`: short text
                   `prty`: priority (high, medium, low) 
                   `srvc`: (routes bulletin applies to) contains list of 
                       `rt`: route designator
                       `rtdir`: route direction
                       `stpid`: bus stop ID number
                       `stpnm`: bus stop name
          
          http://realtime.portauthority.org/bustime/apidoc/v1/main.jsp?section=serviceBulletins.jsp
          """
          
          if not (rt or stpid) or (rtdir and not (rt or stpid)):
              raise ValueError("You must specify a parameter.")   
  
          if listlike(stpid): stpid = ",".join(stpid)
          if listlike(rt): rt = ",".join(rt)
          
          url = self.endpoint('BULLETINS', dict(rt=rt, rtdir=rtdir, stpid=stpid))    
          return self.response(url)