public ActionResult PorContrato(int id)
 {
     if (id == 0)
     {
         return(RedirectToAction(nameof(Index)));
     }
     else
     {
         IList <Pago> lista = repositorio.ObtenerPorContrato(id);
         Contrato     c     = repositorioContrato.Obtener(id);
         if (c == null)
         {
             return(RedirectToAction(nameof(Index)));
         }
         ViewBag.Contrato = c;
         return(View("Index", lista));
     }
 }